Remote Sensing in Soil Science

Developments in Soil Science 15 REMOTE SENSING IN SOIL SCIENCE Further Titles in this Series 1. I. VALETON BAUXITES ...

Author: M.A. Mulders

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Developments in Soil Science 15

REMOTE SENSING IN SOIL SCIENCE

Further Titles in this Series 1. I. VALETON BAUXITES 2. I A H R FUNDAMENTALS O F TRANSPORT PHENOMENA IN POROUS MEDIA

3. F.E. A L L I S O N SOIL ORGANIC MATTER AND ITS ROLE IN CROP PRODUCTION 4. R. W. SIMONSON (Editor) NON-AGRICULTURAL APPLICATIONS O F SOIL SURVEYS

5 A . G.H. B O L T and M.G.M. B R U G G E N W E R T (Editors) SOIL CHEMISTRY. A. BASIC ELEMENTS 5B. G.H. B O L T (Editor) SOIL CHEMISTRY. B. PHYSICO-CHEMICAL MODELS 6. H.E. D R E G N E SOILS O F ARID REGIONS

7. H. A U B E R T and M. P I N T A TRACE ELEMENTS IN SOILS

8. M. S C H N I T Z E R and S. U. K H A N (Editors) SOIL ORGANIC MATTER

9. B.K.G. T H E N G FORMATION AND PROPERTIES OF CLAY-POLYMER COMPLEXES 10. D. Z A C H A R SOIL EROSION

11A. L.P. WILDING, N.E. SMECK and G.F. H A L L (Editors) PEDOGENESIS AND SOIL TAXONOMY. I. CONCEPTS AND INTERACTIONS 1 IB. L.P. WILDING, N.E. SMECK and G.F. H A L L (Editors) PEDOGENESIS AND SOIL TAXONOMY. 11. THE SOIL ORDERS

12. E.B.A. BISDOM and J. DUCLOUX (Editors) SUBMICROSCOPIC STUDIES O F SOILS 13. P. K O O R E V A A R , G . MENELIK and C. DIRKSEN ELEMENTS O F SOIL PHYSICS 1 4 . G.S. CAMPBELL SOIL PHYSICS WITH BASIC

--

TRANSPORT MODELS FOR SOIL-PLANT

SYSTEMS

Developments in Soil Science 15

REMOTE SENSING IN SOIL SCIENCE M.A. MULDERS Department of Soil Science and Geology, Agricultural University of Wageningen, P.O. Box 37, Wageningen, The Netherlands

ELSEVIER

-

Amsterdam

- Oxford - New

York -Tokyo 1987

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211,1000 AE Amsterdam, T h e Netherlands

Distribution for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, NY 10017,U.S.A.

Lihrary of Cnngcss CataloginginPublication Data

Mulders , Michel Adrianus, 1941Renote s e n s i n g i n s o i l s c i e n c e . (Developments i n s o i l s c i e t x e ; 1:) I n c l u d e s b : i i i c g r a p h i e s and i n d e x . 1. S o i l science--Renote s e n s i n g . I . T i t l e . 11. S e r i e s . f5C. .135.Md 19:'i i 31.4 'CLi '7 57-54,3 ISBN 0-444-4,713-X

ISBN 0-444-42783-X (Vol. 15) ISBN 0-444-40882-7 (Series) 0 Elsevier Science Publishers B.V., 1987

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form of by any means, electronic, mechanical, photocopying, recording o r otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./Science & Technology Division, P.O. Box 330,1000 AH Amsterdam, T h e Netherlands. Special regulations for readers in t h e USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. Printed in T h e Netherlands

V PREFACE

The soil scientist is involved in the study of environment since the environmental conditions have

to be evaluated for their impact on soil

formation. However, it may be that the impact of past environmental conditions has

been

of

even more

importance on soil morphology

than the present

conditions. Therefore geo- and morphogenesis form also part of his field of study. His subject of interest is often not visible. In areas covered by vegetation, he has to use combinations of aspects such as natural vegetation or land use and relief to find a clue to the geographical extension of soil bodies. The combinations are not f i x e d but depend on the type of landscape. For example, vegetation may be the effect of human interference and may not at all offer a clue to soil condition. Scientists involved in geographical distribution of soil, especially in medium and small scale surveys, obtain much profit of remote sensing techniques because they offer an overview over large areas and make the study of various landscape elements individually as well as their interrelationship possible. During the past decade, remote sensing techniques developed rather fast. Therefore, it is possible that a soil surveyor becomes old-fashioned by not knowing the potential use of modern techniques. This book is dealing with remote sensing techniques and their application in the field of soil science. It may be used by students and scientists in soil science, geography, geology, hydrology, ecology, agriculture and civil engineering. Basic knowledge of soils, geomorphology, geology and physics will provide useful background. The reader is stepwise introduced to remote sensing by the following subjects:

-

basic physics concerning the interaction of electromagnetic radiation with matter (chapter 2);

-

spectral data of soils, rocks and plants (chapter 3) as a transition of chapter 2 to chapters 9, 10 and 11;

-

technical aspects (chapters 4 , 5, 6 and 7 ) ; interpretation of remote sensing data (chapters 8, 9, 11, 12 and 13).

A guide for reading is presented in the following scheme:

VI Chapters

Subject

solar radiation

Physical aspects

i n t e r a c t i o n processes

Future prospects

The p u r p o s e of

14

t h e hook i s t o p r e s e n t ,

b e s i d e s remote s e n s i n g t e c h n i q u e s and

i n t e r p r e t a t i o n methods a s w e l l a s most

application,

of

the basic parameters

u s a b l e f o r m o d e l l i n g of t h e i n t e r a c t i o n .

ACKNOIJLEDCEHENTS The wide r a n g e of t e c h n i q u e s and a p p l i c a t i o n s made i t n e c e s s a r y t o d i s c u s s v a r i o u s t o p i c s w i t h s p e c i a l i s t s . Thanks a r e due f o r d i s c u s s i o n on p a r t s o f t h e m a n u s c r i p t t o s e v e r a l Dutch c o l l e a g e s . The a u t h o r f e e l s g r e a t l y i n d e h t e d t o D r . Loedeman, G.F.

Ir. E.P.IJ.

Epema and Ir. R .

Attema,

Ir. J . T .

s c i e n c e of

Schurer,

I r . L. Routen, 14.

Ir. J . H .

t e n Rerge, D r s .

Jordens f o r t h e i r suggestive c r i t i c i s m .

Acknowledgement i s made t o c o - o p e r a t o r s grassland

K.

van d e r Veer, D r . 1 r .

of

t h e Department of

f i e l d c r o p s and

t h e A g r i c u l t u r a l U n i v e r s i t y Wageningen f o r t h e v a l u a b l e

i n f o r m a t i o n , t h e y p r o v i d e d on s o i l c o n d i t i o n and f i e l d c r o p s i n t h e i r e x p e r i mental

fields.

Rennema ( -t ) project.

The

However my g r e a t e s t d e b t of

g r a t i t u d e is t o Prof.

Th.

Ir.

linguistic

abilities

of

Drs.

J.M.

de

Zwart

have

van Hummel-Mom

and Mw.

M.H.

van Eldik-v.

M i l t e n h u r g of

been

O.D.

t h e m a n u s c r i p t and t o Mr. P.G.M.

Versteep,, Hr. G .

of

I ' m indebted t h e Depart-

ment of s o i l s c i e n c e and geology of t h e A g r i c u l t u r a l U n i v e r s i t y TJageningen type-writing

J.

f o r h i s encouragement and i n i t i a t i v e f o r s t a r t i n g up t h i s book-

c o n s i d e r a b l e v a l u e f o r a c o r r e c t p r e s e n t a t i o n of t h e E n g l i s h t e x t . t o Mw.

Dr.

for

Ruurman and Hr.

J e r o n i m u s of t h e s a n e d e p a r t m e n t f o r p e r f o r m i n g t h e d r a w i n g s .

VII

CONTENTS PREFACE ACKNOWLEDGEMENTS 1. INTRODUCTION 1.1. Remote sensing 1.2. Concept of soil 1.3. Soil mapping 1.4. Remote sensing in soil science 1.5. Conclusions 1.6. References 1.7. Additional reading

PAGES

2.

12-54 12-16 16-18 18 19-22

3.

4.

INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER 2.1. The nature of electromagnetic radiation 2.2. Radiation laws 2.3. Solar irriadiance and earth emittance 2.4. Concepts of matter 2.5. Atomic-molecular effects on the interaction process; polarization, dielectric constant, refractive index and absorption factor 2.6. Macroscopic effects on the interaction process; a description using the wave model of EMR 2.7. Thermal properties 2.8. Atmospheric effect on EMR 2.9. Energy balance 2.10. Spectral reflectance 2.11. Conclusions 2.12. References 2.13. Additional reading

1-11 1-4 4-5 5-8 8-10 11 11 11

22-26 26-36 36-40 40-45 45-48 48-50 50-52 52-53 53-54

DATA ON INTERACTION OF SHORT WAVE RADIATION WITH NATURAL OBJECTS 3.1. Interaction of short wave radiation with minerals and rocks Spectral reflectance Spectral emissivity 3.2. Interaction of short wave radiation with soils Spectral reflectance Thermal data 3.3. Interaction of short wave radiation with plants Spectral reflectance Thermal properties 3.4. Implications for remote sensing 3.5. Conclusions 3.6. References 3.7. Additional reading

55-60 55-58 58-60 60-75 60-69 69-75 75-86 75-83 83-86 86-87 87-88 88-90 90-91

DETECTION OF ELECTROMAGNETIC RADIATION 4.1. Human vision 4.2. Photographic techniques 4.3. Non-photographic techniques 4.4. Remote sensing from various platforms 4.5. The nature of remote sensing data 4.6. Ground-investigations 4.7. Conclusions

93-124 93-101 101-109 109-112 112-1 15 115-117 117-121 12 1-1 22

55-91

VIII 4.8. 4.9.

References Additional reading

122-123 123-124

5.

PROCESSING OF REMOTE SENSING DATA AND AUTOMATED CLASSIFICATION 5.1. T e c h n i c a l a s p e c t s i n p r o c e s s i n g of p h o t o g r a p h i c imagery 5.2. P r o c e s s i n g of d i g i t a l d a t a 5.3. Information e x t r a c t i o n process 5.4. Automated c l a s s i f i c a t i o n 5.5. Geometrical a s p e c t s 5.6. Conclusions 5.7. References 5.8. Additional reading

125-140 125-131 131-1 35 135-136 136-138 138 138- 1 3 9 139-140 140

6.

IMAGE CHARACTERISTICS R e s o l u t i o n and scale 6.1. Grey t o n e , c o n t r a s t and c o l o u r 6.2. Airphotos 6.3. Images d e r i v e d from l i n e - s c a n n i n g d e v i c e s 6.4. Image-enhancement 6.5. Conclusions 6.6. References 6.7. Additional reading 6.8.

1 4 1-1 5 4 141-1 4 3 143-144 144-148 148-151 151-153 153-1 5 4 154 154

7.

AERIAL PHOTOGRAPHY 7.1. General a s p e c t s 7.2. Stereoscopy 7.3 Aerial mapping cameras 7.4. Photomosaics, o r t h o p h o t o g r a p h s and s t e r e o t r i p l e t s 7.5. Requirements f o r a e r i a l s u r v e y 7.6. True c o l o u r a e r i a l photography 7.7. I n f r a r e d a e r i a l photography 7.8. M u l t i s p e c t r a l a e r i a l photography 7.9. U l t r a v i o l e t photography 7.10. Conclusions 7.11. R e f e r e n c e s 7.12. A d d i t i o n a l r e a d i n g

155- 1 8 0 155-16 2 162-167 167-171 172 172-174 174-1 76 176-177 177-1 7 8 178 178- 179 179-180 180

8.

GENERAL DIRECTIONS FOR PHYSIOGRAPHIC INTERPRETATION OF REMOTE SENSING IMAGERY I N S O I L MAPPING 8.1. Methods of i m a g e - i n t e r p r e t a t i o n 8.2. Landtypes 8.3. R e l i e f , s l o p e and s i t e 8.4. Natural drainage patterns 8.5. Natural vegetation 8.6. Land u s e , c r o p s and p a r c e l l i n g 8.7. Drainage c o n d i t i o n 8.8. Other a s p e c t s 8.9. Conclusions 8.10. R e f e r e n c e s 8.11. A d d i t i o n a l r e a d i n g

ixi-zin 182-186 186-18R 188- 192 192-200 2nn-20 3 20 3- 20 5 20 5 206-208 209 209 210

INTERPRETATION OF AIRPHOTOS FOR SOIL MAPPING AND LAND EVALUATIOfi 9.1. I n t e r p r e t a t i o n of black-and-white a i r p h o t o s 9.2. The legend of t h e a i r p h o t o - i n t e r p r e t a t i o n map From a i r p h o t o - i n t e r p r e t a t i o n map.to s o i l map 9.3.

211-245 211-219 219-222 223-2 26

9.

IX 9.4. 9.5. 9.6. 9.7. 9.8. 9.9. 9.10. 9.11. 9.12.

Land evaluation and planning of field survey Interpretation of true COlOUK airphotos Interpretation of black-and-white Infrared airphotos Interpretation of false colour airphotos Application of multispectral photography Interpretation of sequential aerial photography Conclusions References Additional reading

10. AIRBORNE LINE-SCANNING IN THE 0.3 - 8 um ZONE 10.1. Airborne line-scanners 10.2. Detection in the Ultraviolet 10.3. Detection in the Visible zone and near Infrared 10.4. Detection in the mid Infrared 10.5. Conclusions 10.6. References 10.7. Additional reading 11. REMOTE 11.1 11.2. 11.3. 11.4. 11.5. 11.6. 11.7. 11.8. 11.9.

SENSIITG FROM SPACE IN THE 0.3 - 3 pm ZONE Manned space missions and unmanned satellites Technical aspects Landsat Annotations Landsat MSS imagery Processing and interpretation of Landsat MSS data. Interpretation of Thematic Mapper (TM) data Application Conclusions and comments References Additional reading

226-235 235 235 236-239 239-240 240-24 1 241-242 24 2-24 3 243-245 246-255 246-248 248-249 249-253 253 253 254 254-255 256-287 256-258 258-264 264-266 266-281 28 1-284 284 284-285 285-287 28 7

12. THERMAL INFRARED LINE-SCANNING AND RADIOMETRY IN THE INFRARED

AND MICRC-WAVE ZONES 12.1. Airborne Infrared line-scanners and Infrared imagers 12.2. Satellite programs 12.3. Characteristics of airborne thermal Infrared imagery 12.4. Thermal models 12.5. Interpretation of thermal data 12.6. Application of thermal Infrared line scanning. 12.7. Non-imaging sensing in the Infrared and passive Microwave sensing 12.8. Conclusions 12.9. References 12.10 Additional reading 13. ACTIVE 13.1. 13.2. 13.3.

13.4. 13.5. 13.6.

SENSOR SYSTEMS Laser systems Radar systems Interaction of Microwaves with objects at the earth surface Surface roughness Slope/orientation Dielectric properties Determination of soil moisture Ground penetrating radar Vegetation backscattering Radar image characteristics

288-314 289 289-291 291-295 295-300 300-305 305-306 306-309 309 309-31 1 311-313 314-354 3 14-3 15 315-323 323-332 323-324 325 325-329 329-332 332- 3 34 334-33 5 3 35- 339

X 13.7. 13.8 13.9. 13.10 13.11 13.12

Interpretation of radar imagery Remote sensing with radio waves Applications and future developments Conclusions References Additional reading

339-346 347-348 348-350 350-351 351-353 353-354

1 4 . IMPLICATIONS OF REMOTE SENSING 14.1. Summary on applications 14.2. Land evaluation 14.3. Methodology 14.4. Recent and future developments 14.5. Political and legal considerations 14.6. Education and training 14.7. References 14.8. Additional reading

355-370 355-356 356-357 357-360 360-366 366-367 367-369 369-370 370

Plates 1 - 5 Abbreviations, symbols, units of m e a s u r e INDEX

37 1-373 374-375 37 6- 3 7 9

1 1.

INTRODUCTIOIJ

Ry way of an introduction, the meaning of the term remote sensing and the concepts of soil are discussed. The role of remote sensing in soil science is a logical consequence of these concepts. 1.1.

Remote sensing Remote sensing

OK

teledetection (French: t616dGtection),

sensu strict0

means sensing from a distance, whereby the distance itself is not defined. A well-known form of remote sensing is the use of OUK senses. An example

of a sensing mechanism,

OK

sensor. is the eye, which is sensitive to solar

radiation of a particular wavelength. Looking at an object means sensing the light reflected by

that object.

characteristics (recognition) and

The signals are translated into object into distance.

in'tensity of strong sunlight we can use filters (e.g.

In Order to reduce the sunglasses).

Defects of

the eye may be corrected by the use of optical lenses, while we can observe at a far distance with the aid of binoculars. As

stated before, the distance itself is not defined, therefore, X-ray

machines collecting information from a very short distance as well as radar operating from a l o n g distance can be regarded as remote sensing means. In engineering, a measuring device which collects signals at one place, these being displayed at another place by using radiocommunication, is called a remote sensing unit. In the present text, a remote sensor, is defined as a device collecting data from a distance that varies from a few metres to hundreds of kilometres. The data may be kept in a storable form (e.g. tapes etc).

aerial photographs, magnetic

In contrast to our memory, which is not capable of exactly

recalling past scenery, the stored information enables the user to look simultaneously at various recordings of the scenery of a specific place but recorded at different times. Remote sensing may be executed in various ways, using Electromagnetic Radiation (EMR),

soundwaves

OK

gravity forces. An important part of remote

sensing belongs to the field of study of the geophysicians. For remote sensing of the environment there are three basic aspects:

-

the physical aspects related to the interaction of EMR with objects or features at the earth surface resulting in specific data which can be

2

used for recognition and identification of these objects and features,

-

the morphographic and physiographic aspects related to the appearance of the environment on remote sensing imagery enahling identification and description of objects and features which may be used for a subdivision of land in land-units,

-

the morphogenetic aspects related to the appearance of the environment and the processes that have shaped the land-units (landscape genesis). If

radiation

of

wavelengths

outside

the

Visible

zone

of

the

Electromagnetic Spectrum (EMS) is used, and as a consequence the image is not familiar to the human eye, physical aspects will become more important. For soil science, we focus in this text on remote sensing by E m . The systems used f o r remote sensing may be passive when the EMR available in nature is used, o r

active when the EMR has to be supplied for remote

detection. Various stations are used for remote sensing of the earth (see par. 4.4),

like groundborne platforms

(e.g.

aircraft) and spaceborne platforms (e.g.

towers),

airborne platforms (e.g.

satellites). The wavelength zones of

the EMS normally used for remote sensing may vary from the Visible, the Near Infrared, the Far Infrared (e.g. (see par. 2.1).

thermal infrared) to that of the Microwaves

An active system using Microwaves is radar (chapter

13).

Remote sensing is not a new science, since one of the techniques, aerial photography, has been used for decades. The first aerial photographs were taken from balloons, around 1850 (De Breuck en Daels, 1967). During the

Second World War, considerable experience was gained in

interpreting airphotos for military purposes. The so-called false COlOUK-fflm was

invented for

the detection of

green painted camouflaged tanks and

artillery of the enemy. A t present airphoto-interpretation is an important aid for mapping the natural environment, in particular in less-developed countries and more generally in areas having a low population density. Besides mapping, the modern and more sophisticated techniques of remote sensing are making a number of interesting other applications possible. They may expand our "view" by the use of various devices as well as by the use o f different wavelength zones throughout the EMS. Moreover, a relatively accurate view may be obtained through the application of spectral signatures in combination with shape, size, grade, density and site as diagnostic characteristics of objects and features. Typical for remote sensing research is the multiconcept, which comprises the following:

3 multispectral (or multiband) observation, which is the observation in different wavebands enabling a spectral signature of objects; multistation, which is the observation from different stations at the same altitude

(stereosco.py) or different altitudes (multistage with

different scaies); multipolarized observation used for the study of polarizing properties of objects; multidate

multitemporal) observation, which is the observation of the

(OK

same area or object at different times e.g. in different seasons; in this manner, dynamic features like soil moisture and plant growth can be monitored in the areas under consideration; multi-enhancement or the enhancement of

imagery derived from digital

processing or photographic recording. The extensive application of purposes

remote sensing techniques for military

is linked to the advantages of radar and

nighttime operations and the use of

'thermal' scanning in

satellites for detection from out of

space. Automatic data acquisition has a high priority as a result of the large amount of data to be gathered and processed, and the fact that the information usually has to be available at the shortest possible notice. The difficulties encountered in inventoring and monitoring the natural environment are generally of a more complex nature than those encountered in the military field. Environmental studies are concerned with the identification and understanding of a large variety of natural features and dynamic processes, which are often interrelated in a very complex way. By using remote sensing techniques, we are able to study the interrelations and interactions fixed in the images. The interpretation of

these

images, often

in

close

cooperation with

other

disciplines in order to reveal the underlying basic processes and relationships, will enable us to control and ameliorate the use of the environment. The

application

of

modern

remote

sensing

techniques

and

physics

in

environmental sciences is not an easy task. Once a certain technique is accepted, the use of it might become a habit and only reluctantly will it be replaced. On the other hand scientists might become so mesmerized by the possibilities

of

modern

remote

sensing

established values of the older techniques.

that

they

tend

to

forget

the

4

It is regrettable to state that some of the remote sensing techniques are still in a juvenile stage as regards their methodology, despite the expected potentialities. Therefore in order to avoid disappointment during this stage of the development of remote sensing, it is essential to indicate with care the

best-fitted

and

proven

remote

sensing

technique

for

a

particular

environmental study. 1.2.

Concept of soil The Russian and American concepts of soil are briefly discussed below.

The Russian school developed the following concept of soil: Soils are natural bodies, each with a unique morphology resulting from a unique combination of climate, living matter, earthy parent materials, relief and age of landform. The morphology of each soil, as expressed by a vertical section through the differing horizons, reflects the combined effects of

the particular set of

genetic factors responsible for its development (Glinka, 1927).

Soil as defined in the U.S.

Soil Taxonomy (Soil Survey Staff, 1975) is

"the collection of natural soil bodies on the earth's surface, in places modified or even made by man of earthy materials, containing living matter and supporting, or capable of supporting, plants out-of-doors". Soil according to this definition does not need to have discernible horizons, although the presence or absence of horizons and their nature is of extreme importance to its classification. Soils have many properties that fluctuate with the seasons like temperature, moisture and biologic regimes. The smallest unit of soil is a pedon. It has three dimensions. Its lower limit is the often vague limit between the soil and "not soil" below. Its lateral dimensions are large enough to represent the nature of any horizon and variability that may be present. In practice, the lateral dimensions have to be determined by examination of trenches or digging with a spade or by augering at frequent intervals. The pedon is usually too small to be a practical mapping unit in soil surveys. A larger unit is needed, a combination of pedons or a polypedon, which occurs as a landscape component or natural soil body. This unit is then a mappable feature distinguished from its surroundings on

the basis of

discriminating criteria, which may be parent material, age of landform, relief and

other

soil

forming

factors.

According

to

the

U.S.

Soil

Taxonomy,

differences between polypedons may be related to the nature and arrangement of horizons or the soil as

a

whole e.g.

differences in mineralogy, structure,

5 consistence, texture of subhorizons and moisture regime. Between natural soil bodies there can be transitional zones e.g.

horizons can become thinner at

places and properties can change gradually. This is illustrated in fig. 1.1. In large-scale soil mapping (par. 1.3), exact soil boundaries. normally

In medium-scale

indicated in the centre of

it is often possible to present

soil mapping,

the boundaries are

transitional zones or complexes of

polypedons are presented on the map. 1.3.

Soil Mapping A systematic soil survey comprises the mapping of individual soil units

or polypedons. The maps can be used in the planning of many different forms of land-use and management practices. Basic data of this nature are of particular value in less developed countries in order to make a prediction of the most desirable form of land-use. systematic

A

soil

survey

usually

involves

airphoto-interpretation

combined with systematic field checking on the nature and homogeneity of the soil units. Generally, the upper metre of soil is described. During the course of the survey, the establishment of

the diagnostic criteria of each uniL and a

continued refinement of the mapping legend takes place. If necessary, specific field

investigations (deep

augerings, up

to

4

or

5

m)

are

initiated.

Furthermore, in order to improve the field-observations, soil sampling for laboratory analysis is carried out, s o that earlier estimates can be adjusted, resulting in more reliable future estimates. However, the field-observations are the main bases of soil mapping, since cost is often the limiting factor with regard to laboratory analysis. The

kind

of

information

collected

by

fieldchecks

and

additional

laboratory analysis during the soil survey usually depends on the purpose of the survey. A survey conducted for multiple goals requires information on a broad scale, while for a limited, well-defined aim, only information on a few characteristics of the soils is wanted. According to the Soil Survey Manual (Soil Survey Staff, 1951), a soil map is a map designed to show the distribution of soil types or other soil mapping units in relation to other prominent physical and cultural features of the earth's surface. From a comprehensive soil map, a series of interpretation maps may be derived, showing for example: the suitability of soils for certain

6

Fig. 1.1 Example of a natural soil body assemblage. X soils of the foot slope Y soils of the slope Z soils of the plateau transitional s o i l s X I , Z' with many properties of X or Z polypedons respectively XY, YZ with properties of X and Y, and Y and Z polypedons respectively minor boundary of polypedons indications: major boundary of polypedons u V U brown or red mottles -grey soil matrix due to presence of groundwater

--

--

------

crops, the erosion hazards under defined classes of management, drainage requirements for an optimum production, or the irrigation potentialities of the area. The optimum scale of the soil survey depends on a number of factors (see Soil Survey Staff, 1 9 5 1 ) :

I -

the purpose to be served;

-

the intensity of land use; the pattern of soils;

-

the scale of remote sensing imagery and other cartographic materials available.

The pattern of soils may be so dense that the distribution of soils can only be shown accurately on large-scale maps (e.g.

1:10,000 or 1:5,000).

However,

often a scale of 1:20,000 is sufficient. Difference is made between the scale of field mapping and the publication scale. The former is often reduced two or three times to the publication scale. The minimum dimensions of units that can be shown on the publication map may be given as follows: 25 mm2 for rounded or square forms;

-

2 mm diameter for elongated forms.

Those units that are too small for presentation on the final map can be described under associated soils. To obtain a broad idea about the amount of augerings needed at a certain publication scale, the following rule can be applied.

If aided by

augering(s)

per

remote-sensing-means, it is necessary to have

cm2 map area at publication scale.

1-3

The exact amount is

determined by landscape complexity within the limits given. Besides augerings, there are profile descriptions of soil pits, laboratory analyses, physical field-data, deep augerings and observations with regard to parent material, slope and

topographic position, which present

further evidence on soil

properties and soil geography. There is no generally accepted classification of scales. The following classes are proposed: detailed

1:lO.OOO

scale and larger

semi-detailed

smaller than 1:lO.OOO up

large-scale

to 1:25.000 scale reconnaissance

smaller than 1:25.000 up

(medium intensity) to 1:lOO.OOO

scale

reconnaissance

smaller than 1:lOO.OOO

(low intensity)

up to 1:250.000 scale

exploratory

smaller than 1 :250 .OOO up to 1:500.000 scale

medium-scale

8

smaller than 1:500.000

schematic

scale

The presentation of soil maps is largely dependent on scale. At a largescale, taxonomic units and phases are generally preferred. At a small-scale, a physiographic entry may give more direct information as well as contrasts among regions

so

that broad areas can be viewed as a whole.

In most less developed countries, detailed soil maps are not sufficiently available, and exploratory and reconnaissance maps have to be compiled to point out areas with a high potential, which thereafter have to be mapped in greater detail. In view of this, remote-sensing-means are indispensable tools and morphographic or physiographic descriptions (see par. 8.1) make up often the first entry to the legend of these maps, because:

-

the knowledge of

soil forming processes

is generally too

low

for

indication of taxonomic units,

-

landtypes and landforms determine the landscape performance, that is a daily reality to man; the physiographic maps are readily understood and geographic soil associations can be indicated both at the second and the third level.

1.4.

Remote sensing in soil science Soil science comprises the mapping of natural soil bodies as well as the

study of dynamical aspects. The mapping of natural soil bodies or soil geography is concerned mainly with the more or less permanent properties of soil whereas the study of dynamical aspects regards features such as soil temperature, soil moisture and structural changes e.g.

surface sealing.

Most remote sensing techniques use radiation which shows only a shallow penetration upon interaction with soil, rock and plant materials. By using these techniques, it is only possible to obtain direct information about the surface of soils and rocks or about vegetation covering the soil. Fieldwork is necessary to estimate the properties of the three-dimensional soil profile. Through combinations of interpretation aspects, soil profile properties may be inferred, but of course these suppositions have to be verified by fieldwork. Therefore, it would be a mistake to regard the interpretation of such remote sensing data as decisive for soil distribution without the undertaking of fieldwork. Even

remote

sensing

aids

that

have

a

deeper

penetration

(e.g.

microwaves), or provide data (with thermal Infrared) which are the result of

9

soil physical structure that is not limited to the soil surface alone, do not enable to reveal the complete complex of soil properties. The above emphasis is given to stress the necessity of fieldwork. Besides this, there is the basic physics, which deserves our attention as another aspect of modern remote sensing. Imagery obtained by the use of Visible radiation is familiar to the eye and in fact may be interpreted through direct recognition and identification, which in cases may be followed by deducing the underlying processes. However, other types of radiation, e.g.

UV, IR or Microwaves, may also be used

for image production to visualize certain properties of the earth's surface. A proper understanding of such imagery needs a physical basis focused on the interaction process of the radiation under consideration with the objects and features at the earth's surface. Specific studies require specific remote sensing techniques. The choice as to which remote-sensing-means is to be used, can be determined by four features.

-

the purpose of the study;

-

the scale of the study;

-

the specific characteristics of objects at the earth's surface in the area under consideration;

-

the climatic conditions.

The purpose of the study may be one or more of the following:

-

soil inventory; airphoto-interpretation is a good aid for this purpose at large and medium scales; at small scales, the use of satellite data as well as airphotos is recommended;

-

mapping of

dynamical features, such as

erosion and soil moisture;

multitemporal techniques are required for the study of dynamics;

-

land evaluation; this requires a good insight in natural vegetation and land-use as well as in soil dynamical aspects; airphoto-interpretation and multitemporal techniques are very useful. The scale of the study determines to a great extend the choice of the

most appropriate techniques. For large scale surveys, airborne methods are required, medium scale surveys may be aided by both airborne and spaceborne methods whereas small-scale surveys are served most by the use of satellite-

data. One should realise that the characteristics of the objects at the earth's surface are in fact the most decisive with regard to the choice of remotesensing-means. To illustrate this, three contrasting situations with specific climatic conditions will be considered: ( 1 ) the temperate zone, ( 2 ) the arid, semi-arid and sub-artic zones, and ( 3 ) the tropical rainforest. The

temperate

climatic

zones

are

generally

intensively

used

for

agricultural purpose. The soil is mainly covered by crops or planted forests and in places by semi-natural vegetation. The semi-natural vegetation may show a close relationship with the soil conditions but in case of crops or planted forests, the vegetative cover of soil cannot be regarded as an important key to determine the soil condition. Only locally (on arable land) is the soil surface bare during some period of the year. Therefore, spectral signature of the soil surface generally only offers information on places which are part of a greater unit (the natural soil body).

The dimensions of the natural soil

body have to be determined through a combination of different aspects which is usual in aiKphOtO-inteKpKetatiO~. Arid, semi-arid and sub-artic regions are characterized by a scarce vegetation-cover and bare rock or soils. Spectral signatures of the soil surface may offer valuable information for soil mapping. Up to now, airphotointerpretation in these regions is the most current tool for soil mapping, but there are good possibilities for the application of multispectral remote sensing in improving accuracy and decreasing the amount of fieldwork in mapping of soil. The third situation we want to consider is the tropical rain forest. In these regions, the natural vegetation, together with the aspects of relief, slope and drainage pattern, offers a good key to soil distribution in many places. Modern remote sensing techniques providing for a synoptic view and airphoto-interpretation have proved to be of great value for mapping of soils in these regions. Finally, climatic circumstances may be decisive with regard to the choice of the most appropriate remote sensing technique. When the climatic conditions rule out techniques that make use of short wave radiation (Visible or Near Infrared) due to permanent cloud cover, one should make use of radiation (radar), which can penetrate humid air and clouds.

long wave

11 1.5.

Conclusions The application of various remote sensing aids may reveal different soil

properties and the interpretation units may show a close relation to soil conditions. Unfortunately, ideal remote sensing techniques are limited to research projects., Mostly, one' has to work with means that are basically not intended for soil survey purposes. From this it can be inferred that there is a good reason that one should have knowledge of the applicability of the present great range in remote sensing techniques. Another reason may be found in the advantages arising from the application of different techniques. Knowledge of basic physics is essential for their optimum use. 1.6

References

Breuck, W. de, and Daels, L., 1967. Luchtfoto's en hun toepassingen. E. StoryScientia. P.V.R.A. Gent, 176 pp. Glinka, K.D., 1927. Dokuchaiev's Ideas in the Development of Pedology and Cognate Sciences. In Russian Pedol. Invest. I. Acad. Sci. U.S.S.R., Leninggrad, 32 pp. Soil Survey Staff, 1951. Soil Survey Manual. Agric. Res. Adm. US Dept. of Agric.: 503 pp. Soil Survey Staff, 1975. Soil Taxonomy. A Basic System of Soil Classification Dept. of Agric. for making and interpreting Soil Surveys. U.S. Handbook No 436, 754 pp. 1.7

Additional reading

Barrett, E.C. pnd Curtis, L.F., 1976. Introduction to Environmental Remote Sensing. London, Chapman and Hall, 336 pp. Estes, J.E. and Senger, L.W. (ed), 1974. Remote Sensing Techniques for Environmental Analysis. Hamilton Publ. Cy, Santa Barbara, California, U.S.A., 340 pp. Mulders, M.A., 1977. Application of Teledetection in Pedology. 1-er Colloque PBdologie T616d6tection A.I.S.S. (I.S.S.S.), Rome: pp. 311-324. Reeves, R.G., Anson, A. and London, D. (ed), 1975. Manual of Remote Sensing. Amer. SOC. of Photogramm. Falls Church, Virginia, Vol. I and 11, 2144 PP * Rudd, R.D., 1974. Remote Sensing. A better View. Duxbury Press, North Scituate, Masachussetts, U.S.A., 135 pp.

12 2.

INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER

Energy can occur in different forms e.g.

kinetic, potential, mechanical,

chemical, electrical and thermal energy. Ocean waves make themselves manifest by their way of propagation. The waves are due to a disturbance at the air-water interface. They are transverse, that is the vibration of the particles is perpendicular to the direction of the propagation. A number of aspects connected with wave motion becomes visible when observing these waves, such as direction, wavelength, amplitude, velocity and frequency

.

Electromagnetic radiation (EMR) is energy that propagates through vacuum (free space) or through material media in the form of an advancing interaction between electric and magnetic fields.

It can make itself manifest by its

interaction with matter. Light and thermal energy are examples of EMR. Besides by radiation, thermal energy may travel by conduction and convection. In this chapter, physical concepts of EMR and its interaction with the atmosphere and objects at the earth's surface are discussed. The interaction process is of great importance to the remote sensing specialist. 2.1.

The nature of electromagnetic radiation The properties of EM waves can be summarized as follows (see Fig. 2.1):

-

the waves are transverse; the electric (E) and magnetic ( H ) vectors are perpendicular to the direction of propagation, mutual perpendicular and in phase. EM waves can be characterized by wavelength, amplitude, phase, frequency,

direction, velocity, polarization and coherence of the radiation. EMR which has a fixed direction of the electric vector is said to be plane polarized. Polarization of light can take place upon interaction with matter. The coherence of waves concerns the relationship of phases; coherent waves or uniform plane waves have a regular or systematic relationship between their phases, while incoherent waves have phases that are related in a random fashion. Radiant energy of natural sources is normally incoherent, however, some artificial sources, such as radar and lasers, are constructed to produce coherent radiation. Interference in a point

OCCUKS

when the EM field in that point is made up

out of contributions from more than one coherent source. When the distances

13

Fig. 2.1 Electric ( E ) and magnetic ( H ) vectors of an EM wave. the waves have travelled differ by a whole number of wavelengths, there will he a maximum of intensity. When the distance transversed differs by odd multiples of a half wavelength, the two waves will exactly cancel each other. Interference may occur when reflections of the surface and of an interface meet each other. A surface illuminated by laser light looks grainy and seems to sparkle.

As the waves are scattered from neighbouring points on the surface, they interfere with one another and reinforce one another if in phase, or cancel one another when out of phase. The interference pattern depends on the angle at which the surface is viewed (Schawlow, 1968). Ordinary light does not produce such interference, because the light waves are unrelated to one another as to phase. The polarization of a uniform plane wave refers to the time varying hehaviour of the electric vector field at some fixed point in space. Consider a uniform plane wave travelling in the z direction with the E and 11 vectors in the x-y plane (Fig. 2.1).

If Ex

= 0

and only Ey is present, the

wave is said to he polarized in the y direction: a similar statement holds for polarization in the x direction. If both Ex and Ey are present and in phase, the resultant electric vector will have a direction dependent on the relative amplitudes of Ex and Ey. The direction of the resultant vector is constant with time and the wave is said to be linearly polarized. If E, and Ey are not in phase, that is, if they don't

reach their maximum

values at the same time, then the direction of the resultant electric vector

14 will not be constant with time. In the particular case where Ex and Ey have equal magnitudes and a 90-degree phase difference: the wave is said to be circularly polarized. Other out of phase cases, produce elliptical polarization (Jordan et al., 1968). The generation of EM waves occurs in wave trains or bursts of radiation. Each wave train, elementary quantum or photon, carries a radiant energy ( Q in of the wave, so that

J) which is proportional to the frequency (f in s-')

J

where h is Planck's constant with a value of 6.626 x The EM waves travel through vacuum at a fixed velocity (c

=

S.

2,998 x lo8 m 6 - l ) .

The general relationship between velocity (c in m s-l) wavelength ( A in m) and wave frequency (f in s-l) is: (2 - 2)

c = f A Combining 2

-

1 and 2

-

2 results in

Q=h' x

(2

-

3)

Consequently, the energy of a photon is proportional to the frequency (2

-

2), and inversely proportional to the wavelength (2-3). The processes involved in the generation of EMR produce radiant energy with specific photon energy, frequency and wavelength. These quantities provide a scale for the so-called electromagnetic spectrum (EMS).

Particular zones are

essential for life, e.g. the Visible zone and the Infrared, or are made use of for practical reasons, e.g. Microwaves and Radiowaves (see Fig. 2.2). The radio spectrum is also indicated in Fig. 2.2. Part of it, that is from Very

- High

-

Frequency (VHF) up to Extremely

-

High

-

Frequency (EHF) is used for

radar and is called Microwaves. For subdesignations of this zone, the reader is referred to chapter 13.

15

g E

!5

v

v

E

F

E

S

E

E

x

m

0

m

0

m

m

m 0

m 0

m

m 0

o m

I

1

I

I

1

1

1

1

I

0

0

E 1

a

0

E m I

E

E m

U m

I

1

0

E

m

E O

E

m 0

m 0

1

1

1

E x

E x

E x

m

m 0

0 0

m

I

1

J

wavelength (m)-

w; N

N

N

N

N

N

N

N

N

I

I

I

I

I

I

I

I

0

0

0

0

0

0

0

0

7

c

c

c

c

7

c

c

I

1

I

1

1

I

1

I

N 0

!

?

E

t

"

"

S

"

I W

N I W

0

-

N I N W

N

I X

I

w

0 0

0 0

0

-

7

c

c

W

W

3

-

:

o

I

N

-

N

I N

c

N

Y

I Y

" E o

0

0

_

_

0

_

N I

Y

0

c

>

-

7

_

-1

300 30 3 300 30 3 300 30 3 GHz GHz GHZ MHz MHz MHz KHz KHz KHz

H = high L = low E = extremely

Fig. 2.2

U = ultra V = very M = medium

l!b

3bf;8;01 ;51;)0 3343 60

3iO km

1

1

1

109876 5 4 3

2

1 KHz

1 1 1 1 I

1

I

The electromagnetric spectrum Abbreviations: V,B,G,Y,O,R = violet, blue, green, yellow, orange, red respectively

16 The Visible zone is subdivided acc. to Weast ( 1 9 7 4 ) into the following bands : COlOUK

Wavelength in nm

Wavelength in nm representative for COlOUK 410 470 520 580 600 650

400-424 424-491 491-575 575-585 585-647 647-700

violet blue green yellow orange red

However, the eye shows a low sensitivity outside this zone respectively down to 380 nm in the short wavelength range, and up to 7 8 0 nm in the long wavelength range (Schurer et al.,

1 9 8 0 ) Therefore, the Visible zone is often

extended. 2.2.

Radiation laws All bodies with temperatures above absolute zero emit radiant energy.

The radiation laws use the concept of a perfect absorber and radiator, the so-

. These laws are:

called black body

-

the Stefan

-

Boltzmann's

law, which states that the total of

emitted from a black body (Me in Wm-')

radiation

is proportional to the fourth power of

its absolute temperature (T in K) according M e = o T4

where a

(2

=

2

15 c

in which c

=

k4 =

TI

2

h

5.7

4)

10-8 wm-2 K-4

3

velocity of light in m s-l

h = Planck's constant (see 2

-

1)

k = Boltzmann's constant = 1.38 x

-

-

5K-l

Kirchhoff's law; since no real body is a perfect emitter, the real emittance

(M) of a radiator is a fraction of the emittance of a perfect radiator (Me), thus

17

where

E

the emissivity (M/Me) of the real body, has a value between 0 (white

body or perfect reflector) and 1 (black body);

-

the wavelength, which

is

correlated to the maximum radiant emittance of the black body ( A max),

Wien’s

displacement law;

this

states

that

is

inversely proportional to its absolute temperature T according to:

Where C 3

=

2898

IJ

m K; the equation indicates that as the temperature

increases, the dominant wavelength of the radiation emitted shifts towards the short wavelengths (see F: :. 2 . 3 ) ;

-5 I

lo5 lo4

E ul

lo3

4J

+

9

v

lo2

c

V W

::.lo-:

U.

0 Wavelength (pn)

Fig 2.3

-

The spectral radiance of a blackbody (after Higham e.a. from Jamieson et al., 1963)

1973, adapted

Planck’s law. describes the spectral relationships between temperature and

radiative properties of a black body when thermodynamic equilibrium exists;

18 the law may be expressed by 1

M dX= 2 X

where M

X

h c2 X

TI

dX

=

(2

radiant energy in Wm-2 within a unit range of d X

h

=

Planck's constant (see 2 - I ) ,

c

=

velocity of light m s - ' ,

k

=

Roltzmann's constant (see 2

A =

wavelength in m,

T

absolute temperature in K,

=

-

-

J)

,

4).

e = base for natural logarithms. The equation enables the assessment of proportions of total emittance for a range between selected wavelengths. 2.3. Solar irradiance and earth emittance The spectrum of solar irradiance (radiant power received per unit area) outside the earth's atmosphere resembles that of a 6000 K black body spectrum, while the spectrum of terrestrial emittance (power emitted per unit area) approximates the 300 K black body. There is a global equilibrium between heat gained from the sun and heat lost to space. Fig. 2.4.

shows the EMS of solar and terrestrial radiation. Solar radiation is

significantly attenuated by the earth's atmosphere. The spectrum of solar radiation at the earth's

surface is in fact a

transmission spectrum, since part of the radiation i s specularly reflected, scattered, or absorbed by the molecules present in the earth's atmosphere. The total solar irradiance arriving at the earth's surface can be divided into a direct and a diffuse component. The latter is the result of scattering by atmospheric aerosols and molecules and will vary with the visual range in spectral properties and intensity. Most of the solar radiation which reaches the earth's surface has wavelengths shorter than 4.0 urn, whereas the radiation emitted by the earth is found mainly in the band 4.0

-

40 um (infrared).

Maxima in solar radiation and

terrestrial radiation are found respectively at 0.5 and 10 um. 2.4. Concepts of matter In remote sensing of the environment, many different types of matter are encountered, these being:

19

-I

uv

VIS I

body i r r a d i a n c e a t 6000 K

2000

E

I

1000 I

I / I

x

v ~

-.

:xtraterrestrial solar irradiance

a cu I E

-

Infrared 1

500

Direct beam (normal incidence) Solar i r r a d i a n c e a t the e a r t h ' s surface

I

I I

0, L

W

w C

200

100

50

20

7

5

2

t -.

Estimated infrared emission

-.

A

Di f f u s e solar irradiance a t the earth's surface

10

1 0.7 0.1

lack body emittance 300 K

-.

'\, I

-.

/ Absorption band

-.

0.2

0.5

1.0

2.0

5.0

10

20

50

100

Wavelength (pm)

Fig. 2.4 Electromagnetic spectra of solar irradiance and terrestrial emittance (modified after Barrett and Curtis, 1976, originally Sellers, 1965)

-

diatomic gases (e.g.

02, N 2 ,

CO);

- polyatomic gases (e.g. H 2 0 , C02); - complex molecular organic materials (vegetation and soil organic matter);

-

solid inorganic substances (minerals).

To understand the interaction of EMR with matter, we need to go down to the atomic and molecular level. If external energy is available, the atoms of a gas may become ionized that

20

is, electrons may be freed from an atom, leaving it as an ion with a net positive charge. On the other hand, electrons may attach themselves to an atom, thus forming a negative ion. In a metal, some of the electrons are free to move from one atom to the next giving rise to the conduction of electricity (Jordan et al., 1968). Substances that contain few or no free charges and consequently are poor conductors of electric current, are called dielectric substances. A good dielectric is one in which the absorption of electric energy is a minimum: a vacuum is the only perfect dielectric (Reeves, 1 9 7 5 ) . In structures such as molecules, the energy states give rise to specific features. Although the number of energy states increases with the density of the structure, that is from gas over liquid to solid, a general understanding may be obtained from the energy states of molecules, such as in gases. Molecules in gases possess three types of internal energy states: rotational, vibrational and electronic. For any electronic state, a variety of vibrational states is possible, and for any vibrational state a variety of rotational states is possible. The total internal energy of the molecules at any time is the sum of the energy of the three states. Electronic states are separated by energy differences corresponding to the energy carried by photons in the UV, blue and green regions; pure vibrational states by

photon energy in the yellow, red and near

Infrared and pure

rotational states by middle - far Infrared and Microwave portions of the EM spectrum (Lintz and Simonett, 1 9 7 6 ) . In liquids, the atoms or molecules are in continual motion but there is a certain amount of ordrr extending over a relatively short distance.

In solids unrestricted motion is not possible and the molecules are fixed in position in an orderly arrangement. The basic unit is

a

crystal.

Because rotational energy states are precluded in liquids and solids, only vibrational and electronic states remain. A number of fundamental vibrational modes are:

- the OH, S i - 0, Si - the H - 0

-

11

-

0

-

Si, Al - 0 - Si and Fe -

0

stretching modes,

and A1 - OH bending modes.

The modes of vibration show allowed frequency bands separated by forbidden regions. Crystals possess a long range order, a periodicity of structure. The forces acting between the a t o m in crystals are determined by the way in which the outer electrons of the composing atoms are distributed in space. One may

21 distinguish between the following extreme types (Dekker, 1958) : a. ionic crystals (e.g.

NaCl),

b. valence crystals (e.g. diamond C.

,

metals (Cu, Ag),

d. van der Waals crystals (many organic crystals). In ionic crystals, one or more electrons of one type of atoms are transferred to another, leading to the formation of positive and negative ions. The cohesive energy is provided mainly by Coulomb interaction between the heterogeneous ions. At elevated temperatures these crystals exhibit ionic conductivity. Ionic crystals are characterized by a strong absorption in the Infrared. In valence crystals, the neighbouring atoms share their valence electrons under the formation of strong bonds.

They are very hard and show a poor

electrical and thermal conductivity. In metals, the outer electrons of the atoms have a high degree of mobility to which these materials owe their high electrical and thermal conductivity. The cohesive energy is provided by Coulomb interaction between the positive ion and the "negative smeared out charge" of the conduction electrons. Molecules like H20 and HC1 may be considered to consist of two ions. They possess a permanent dipole moment which is equal to the effective charge per ion, times the separation of the ions. The interaction between such permanent dipoles provides, besides other forces, the cohesive energy in van der Waals crystals such as organic crystals. In this case, the other forces refer to the socalled dispersion forces which are due to fluctuating dipoles by combination of moving electrons and the nucleus of an atom. The weakness of the forces is expressed by the low melting point of these materials. Between the extreme groups, there are many intermediate ones e.g.

semi-conductors, being intermediate between valence crystals and

metals. To

understand

these

intermediate

groups,

the

electron

orbits

are

considered as energy levels separated by energy gaps. Crystals for which a certain number of energy levels are completely filled with electrons, the other (dielectrics).

levels being

completely

empty, are insulators

On the other hand, a metallic character is caused by the

presence of an incompletely filled energy level. If the energy gap between the empty and the filled energy levels is low, as for intermediate groups, an insulator may become a semi-conductor by thermal

22 e x i t a t i o n . When t h i s happens, some of t h e e l e c t r o n s of t h e upper f i l l e d l e v e l a r e e x i t e d i n t o t h e n e x t empty l e v e l and c o n d u c t i o n becomes p o s s i b l e (Dekker,

1958). I n normal l i q u i d s o r g a s e s ,

t h e o r i e n t a t i o n of

and t h e p h y s i c a l p r o p e r t i e s become independant of

t h e m o l e c u l e s i s random

t h e d i r e c t i o n a l o n g which

t h e y are measured; t h e y a r e i s o t r o p i c . The p h y s i c a l p r o p e r t i e s of s i n g l e c r y s t a l s i n g e n e r a l depend on t h e d i r e c t i o n a l o n g which t h e y a r e measured r e l a t i v e t o t h e c r y s t a l a x e s . T h i s phenomenon i s c a l l e d anisotropy.

Trigonal,

t e t r a g o n a l and hexagonal systems a r e o p t i c a l l y

u n i a x i a l , b e i n g i s o t r o p i c f o r t r a n s m i s s i o n p a r a l l e l t o t h e p r i n c i p a l a x i s of symmetry. Orthorombic, m o n o c l i n i c and t r i c l i n i c systems a r e o p t i c a l l y b i a x i a l . However, c u b i c systems a r e o p t i c a l l y i s o t r o p i c . Besides

s o l i d matter,

rocks

may

also

c o n t a i n l i q u i d s and

s o i l s , t h e p r e s e n c e of w a t e r and a i r i s a must.

gases.

For

The s o l i d s i n r o c k s and t h e

rock fragments o r m i n e r a l s i n s o i l s may range from l a r g e b l o c k s w i t h e i t h e r smooth, p o l i s h e d s u r f a c e s , o r rough s u r f a c e s , t o s m a l l p a r t i c l e s t h a t v a r y i n s h a p e and packing. The macroscopic p r o p e r t i e s , as w e l l as t h e a t o m i c - m o l e c u l a r - and l a t t i c e s t r u c t u r e s , d e t e r m i n e t h e i n t e r a c t i o n w i t h EMR.

2 . 5 . Atomic

-

molecular e f f e c t s

on

the

i n t e r a c t i o n process:

polarization,

d i e l e c t r i c c o n s t a n t , r e f r a c t i v e i n d e x and a b s o r p t i o n f a c t o r . Since m a t t e r i s l a r g e l y made up of charged " p a r t i c l e s " ,

external e l e c t r i c

and magnetic f i e l d s must e x e r t some k i n d of i n f l u e n c e . This i n f l u e n c e w i l l be p r e s e n t whether

t h e p a r t i c l e s a r e f r e e t o move about o r a r e t i g h t l y hound

together (Jordan et al., In

t h e c a s e of

1968).

good c o n d u c t o r s such as m e t a l s ,

t h e e l e c t r i c f i e l d of

the

i n c i d e n t wave c a u s e s c o n d u c t i o n c u r r e n t s t h a t produce t h e i r own e l e c t r i c f i e l d and g i v e r i s e t o v e r y s t r o n g r e r a d i a t i o n of t h e EM wave ( m i r r o r

-

effect).

D i e l e c t r i c s o r i n s u l a t o r s d i f f e r from c o n d u c t o r s , i n t h a t they c o n t a i n no f r e e charges, ions,

atoms,

but

r a t h e r c h a r g e s which a r e t i g h t l y bound t o g e t h e r

p a r t i a l molecules

e x t e r n a l EM f i e l d

causes a

and molecules.

s m a l l but

The a p p l i c a t i o n of

s i g n i f i c a n t s e p a r a t i o n of

t o form a

steady

t h e bound

c h a r g e s , s o t h a t each i n f i n i t e s i m a l element of volume behaves as i f i t were a n e l e c t r o s t a t i c dipole.

The induced d i p o l e . f i e l d t e n d s t o oppose t h e a p p l i e d

field (polarization). On a n atomic s c a l e , e l e c t r o n i c p r o c e s s e s a r e i m p o r t a n t , and c h a r g e s e p a r a t i o n

23

can occur due to the displacement of the negative electron cloud relative to the positive nucleus; this is called electronic polarization. On a molecular scale, vibrational processes (par. 2.4) with atomic or ionic and orientational polarization are important. Atomic or ionic polarization results from the displacement of atoms or ions within molecules (due to an external field). Orientational polarization arises in materials whose molecules are permanently polarized but randomly oriented; an external EM field causes the molecules to align themselves. On a still larger scale, one encounters space-charge polarization.

In that

case, free conduction electrons are present, but are prevented from moving over relatively great distances by barriers such as grain boundaries. The application of an external EM field results in piling up of these electrons against these barriers producing the separation of charge required to polarize the material (Jordan et al., 1968). For each type of polarization there is a typical resonance frequency, which has the precise radiant energy (h.f see 2-1) needed for the energy transition. Material properties important for the interaction are the permittivity or dielectric constant

(E

) and the conductivity ( u ) .

The dielectric constant of a medium is defined by the equation (Weast ed., 1974) :

where F is the force of attraction between two charges Q and Q' separated by a distance R in a uniform medium (capacitance of a capacitor with specific dielectric material).

where

E

Often

E~

is used, the relative dielectric constant:

is the dielectric constant of free space (8.859 x lo-"

The dielectric constant is

a

V-'

A s m-')

measure for the amount of polarization upon

interaction. To obtain normalization, so-called static

E~

values may be derived

by application of a static EM field (frequence zero). The static cL. values of most dielectric media are between 1 and 6 , but water having dipole molecules has an exceptionally high value of about 80 (Prins,

24

1955). Temperature, p r e s s u r e and composition have impact on t h e

E

v a l u e s . So

does t h e f r e q u e n c y of dynamic o r a l t e r n a t i n g f i e l d s . t h e frequency i s low ( e . g .

If

a t Microwave f r e q u e n c i e s and Radiobands),

t h e m a t e r i a l behaves e i t h e r l i k e a conductor o r semi-conductor, f r e q u e n c i e s i t behaves l i k e a d i e l e c t r i c (e.g. The

p o l a r i z a t i o n of

while a t high

I n f r a r e d and V i s i b l e ) .

e l e c t r o n s may be a t t a i n e d

simultaneously with an

i n s t a n t a n e o u s EM f i e l d b u t t h e o t h e r t y p e s of p o l a r i z a t i o n r e q u i r e r e l a t i v e l y much t i m e

t o a t t a i n t h e i r s t a t i c value.

Dekker

(1958) mentions a t i m e f o r

d i p o l a r p o l a r i z a t i o n which v a r i e s between days and of

S.

S i n c e t h e masses

the microscopic bodies c o n t r i b u t i n g t o the p o l a r i z a t i o n increase i n t h e

range of e l e c t r o n s , i o n s , m o l e c u l e s up t o complex m a t e r i a l s l i k e g r a i n s , t h e resonance

frequencies

for

these

successive

polarization

effects

have

to

d e c r e a s e . This i s , because i n e r t i a of t h e p a r t i c l e s becomes r e l a t i v e l y g r e a t and c o n s e q u e n t l y t h e p a r t i c l e s a r e u n a b l e t o

f o l l o w r a p i d o s c i l l a t i o n s of t h e

applied field. By way of i l l u s t r a t i o n , E~ v a l u e s a t r a d i o f r e q u e n c i e s a r e g i v e n i n t a b l e 2.1. Most m a t e r i a l s show v a l u e s between 1 and 14 w i t h i n t h i s f r e q u e n c y range. For example, sodium c h l o r i d e h a v i n g a n

gr

of 6.12

a t VLF shows a lower v a l u e i n

the Visible ( E = ~ 2.25 acc. t o P r i n s , 1955). Its resonance f r e q u e n c i e s are i n Table 2.1.

R e l a t i v e d i e l e c t r i c c o n s t a n t of s o l i d s a t r a d i o f r e q u e n c y and a t T 17 - 22" C u n l e s s s p e c i f i e d d i f f e r e n t l y (Weast ed., 1974).

=

solids apatite

1 11

It

optic axis 11

frequency

€1.

VHF 3 x lo8

9.50 7.41 5.5 8.0 6.8 14.2 86 170 12 5.3 4.34 4.21 8.50 8.00 5.66 6.12 7.10 6.3

I,

11

diamond dolimite 1 optic axis 11

I,

I,

108 11 I,

11

I,

f e r r o u s o x i d e (15°C) rutile 1 optic axis 11

,I

I,

11 11

1,

z i r c o n 1 ,I1 o p t i c a x i s sodium c a r b o n a t e (10H20) quartz 1 optic axis 11

11

It

11

It

It

VLF

1 I

I'

104 I,

11

gypsum sodium c h l o r i d e tourmaline 1 o p t i c a x i s It

HF

11

calcite I optic axis I,

It

6 x lo7 3 x lo7

11

11 11 11

11

25 the Infrared. The relatively heavy ions involved cannot follow the rapid ossillations in the Visible. The same applies for frequencies in the Visible,

E

of water for the high

being around 1,77.

E

r In an electric field, the frictional work done in polarization of atoms

and molecules absorbs energy from the field. When the field is removed the orientation is lost by thermal agitation. If free charge carriers are present, the current also causes heat loss because of the resistance of the material. At high frequencies (Infrared, Visible and UV), the losses are relatively high, only electronic or ionic (or atomic)

polarizations are active and

permanent dipoles are not able to follow the field variations. At low frequencies (radio and audio),

the losses are relatively small and

permanent dipoles contribute their full share to the polarization. Another term used to describe the interaction of EMR with matter is the index of refraction ( n ) which indicates the ratio of the velocity of EMR travelling through free space (c) to its velocity in a specified medium (A) n = -nA n= C

(2

-

10)

vA where n A is the refractive index of the specified medium, and no that of free space. For the same frequency,

E

of dielectrics is related to n as follows:

(2

Er=n2

-

11)

Normally, the EM wave experiences exponential damping in traversing the material. The damping in electronic, atomic or ionic displacements is caused by the restoring and frictional forces. Although for electronic polarization in a specific case with low damping, absorption and emission at a critical frequency may be produced simultaneously (selective reflection acc. to Jenkins et al., 1957), absorbed.

generally, a considerable portion of the incident radiation is

The radiation absorbed is converted into radiation of a lower

frequency and a longer wavelength: mainly thermal radiation. Lambert's law relates the original intensity (10) to the intensity (I) after passing through a thickness x of a material with an absorption factor k:

I

=

Ige-kx

(2

-

12)

The wave may penetrate only a very short distance in good conductors

26

before

being

reduced

to a negligible

small percentage of

its original

strength. The depth of penetration 6

or skin depth is defined as that depth in which

the wave has been attenuated to l/e or approximately 37 percent of its original value. At that distance kx

=

1 (see 2

-

12) and since 6

=

x in that

-

13)

case 6s = - 1

(2

k

For absorbing media, the index of refraction does not properly describe the interaction process. One has to use a loss term in addition. This is expressed by the complex refractive index (it),

which is given by:

where k is the absorption factor (2 - 12), and j= From this relation together with 2

-

11,

a.

it follows for absorbing media

that

where

E

is the complex dielectric constant ( X,T)

with

defining velocity and wavelength in the material, and

E' E"

as the real part is the imaginary

part which expresses the energy losses in the medium.

E"

and

E"

=

2nK or n

=

2K

(2

-

15)

(2

-

16)

2.6. Macroscopic effects on the interaction process; a description using the wave model of EMR. In section 2.5,

the displacement of charges (polarization) is introduced.

The EM field which causes the polarization is an alternating field. The displacements are elastic and have a restoring force which is proportional to the displacement itself. They can be treated as harmonic oscillations. The external EM energy is in resonance when its photon energy is equal to the

27 difference between two energy levels of the atoms or molecules in the target. Actually, the atoms

OK

molecules also react to EMR of any frequency: the

socalled nonresonant reaction. The molecules and atoms act as oscillators if an EM wave passes over them. The EM wave induces a vibration of the oscillator in

the target, so that it oscillates with the frequency not with its own resonance frequency

wo

w(=2nf)

of the field and

*

In principle, a vibrating charge is an emitter of E m . In gases, there is an individual incoherent scattering by each oscillator. There is no particular interference among the oscillators of gases. However, the molecules or atoms of solids (and of liquids or even cloud droplets) show an orderly arrangement, which results in interference of the reemitted waves. The interference is destructive in all directions except for the forward direction where it is constructive. In the forward direction, the reemitted waves build up to a single refracted wave. This is not so near the surface of the material. There is a thin layer (about $ A thick) of oscillators at the surface for which the back radiation is not completely

canceled

by

interference.

The

oscillators adds up to a reflected wave.

radiation "backward"

of

these

The intensity of the reflected

radiation is proportional to N2, where N represents the number of oscillators producing radiation waves which are in phase at a given point in space 4 (Weisskopf, 1965; N2 is proportional to A ). Macroscopic

effects

such

as

true

surface

or

specular

reflection,

scattering and refraction take place at boundaries and are a function of chemicophysical

structure

as

well

as

roughness and

orientation

of

the

boundaries.

To obtain a simple model, EMR is assumed to travel from a less dense medium (air) to a more dense medium with a plane surface. Upon interaction, part of the EMR is reflected from the surface, the rest enters the substance and is transmitted to a degree depending on the absorption characteristics. The angle Bi formed by

the direction of

propagation of

the incident

radiation with the normal to the surface (see Fig. 2.5) differs from

Or the

angle of refraction, which is the angle formed by the direction of propagation of the radiation penetrating the substance with the normal to the surface. Fig.

2.5 presents specular reflection, which occurs when surfaces are smooth

28

i nc i den t ra dia tion

r e f 1 ec t ed r a d i a t i o n

I

J

o p t i c a l l y l e s s dense layer ( A )

o p t i c a l l y more dense layer (B)

transrni tted/ radiation

Fig. 2.5

I

a

o p t i c a l l y l e s s dense 1 a ye r

Specular reflection, refraction and transmission of light.

a) general situation b) specific case: angle between reflected and refracted rays is 90" -parallel to plane of incidence (vertical polarization). 0 perpendicular to plane of incidence (horizontal polarization). Plane of incidence: plane formed by the normal to the surface and the direction of propagation of the incident wave. Horizontal and vertical refer to the plane surface upon which the wave is incident, although the direction can only be near vertical at grazing angles.

29

and

highly

polished.

It follows Snell's

law, which gives

the index of

refraction (n) relative to free space for each medium. When the media A and B are concerned: n

sin 0

n

sin 0

B - _

A

i

2.6.

This relationship is expressed in Fig. derived from this figure: at low angles of

Two

conclusions may be

incidence ( O i

=

10" - 2 0 " ) a

difference in the index of refraction does not cause great differences between O i and

0, (low O i

angles (Oi

=

-

Elr);

Oi

-

0, becomes very high for high n at grazing

70" - 8 0 " ) .

8ol

/

n = l ,O

n=l,l

n=l,3 n=l,5

n=2,0 n=2,7

Fig. 2.6 Relationship between

Oi, 0, and n (n=

nB

- ),

nA The horizontally polarized components of the incident EMR (see Fig. 2.5b) parallel to the interaction surface do not meet

discontinuities in that

surface and therefore are strongly reflected. The Fresnel reflection factors apply to flat smooth boundaries between two homogeneous and isotropic media, coherent monochromatic EMR and a medium in which multiple reflections do not occur. They have the following form:

30

-

E~

n2 cos e

+

Jn2-

= q =n2-cos ei + A'

Rv

'T E~

=

where

\

=

=

sin

-

2

ei

(2

-

18)

sin2 ei

2

cos

ei -

Jn2

-

sin

cos

ei +

Jn2

-

sin2 ei

ei

( 2 - 19)

reflection factor for vertical polarization,

Rh

=

reflection factor f o r horizon polarization,

Ei

=

amplitude of incident electric field with v

E,

=

amplitude of reflected electric field with v or h polarization,

0

=

angle of incidence,

=

index of refraction ratio between two media,

n

i

In table 2.2 equations 2

-

some values are given for

18 and 2

Table 2.2 Values of p n

oi

'G 200 40" 60" 80"

i

-

and Rh calculated according to

= 1.5

-0.104 -0.252 -0.645 -0.795

OK

n

= 1,5

and n

= 2.

n = 2 ph

%

PV

0.012 0.064 0.416 0.632

-0.311 -0.235 -0.518 0.431

0.097 0.055 0.268 0.186

Rh

0.083 0.021 0.012 0.237

h polarization

19.

and ph at Oi 2 0 , 4 0 , 6 0 and 80"

PV

-0.288 -0.146 0.112 0.487

\

OK

Ph

Rh -0.354 -0.424 -0.565 -0.819

0.125 0.179 0.319 0.671

In table 2.2 the reflectance for horizontal polarization:

ph

= (Rh)2

=

Irh where

Irh is intensity of reflected radiation with

Iih horizontal polarization, and

Iih is intensity of

horizontal polarization (item

p v).

incident radiation with

The negative signs indicate phase changes.

These, however, are immaterial for the intensities since the latter are dependent on the squares of the amplitudes. When we consider the intensity figures of table 2 . 2 ,

we see that for

vertical polarization with increasing angle of incidence an angle will be reached where polarization

the intensity becomes zero, this in contrast to horizontal the

intensity

of

which

increases

with

increasing angle

of

31 incidence. The angle at which vertical polarization is lowest, and therefore the reflected radiation is best polarized, is called the Rrewster angle. When this situation occurs Snell's law (2

-

17) can be written as:

sin B i

nB -=

-

sin (90

nA

= tan 0

0,)

(2-20)

i

The situation is presented schematically in Fig. 2.5b. Fig.

2.7a

shows

polarization ( pv) medium with n at about 56'.

reflectance

of

and

horizontal( ph )

vertical

in relation to the angle of incidence for a dielectric

1.5.

=

the

The polarizing angle or Brewster angle (tan Oi

From the data in Fig. 2.7a,

1.5)

=

is

it may be concluded that upon

interaction at Oi larger than 3 0 " , the reflected EMR shows a predominance of horizontally polarized radiation (surface reflection).

1 .o

f ref1 ec tance

1 .o

+

reflectance

t

b.

P. I-

1

angle of incidence Brewster angle

1 9n0

I

I

I

I

'

1

I I

'

I

angle of incidence

'

h

goo

Fig. 2.7 Reflectances (Higham et al., 1973) (a) non-absorbing medium (b) absorbing medium Fig. 2.7b shows

ph and p

for an absorbing medium. It appears that the

reflectances are higher than for the non-absorbing medium and that

pv

does

not become zero at the Brewster angle. So

far, targets with a plane surface and coherent radiation have been

considered.

It will be evident that granular materials (e.g.

soils) and

32 incoherent radiation offer more complications. Besides reflection at the surface, internal reflection

OCCUKS

at surface

boundaries within the granular material. The internally reflected ray leaves the grain upon refraction at the grain surface and is added to the total reflected radiance. Therefore for incoherent illumination, the total reflected radiance of a granular object is thought to be composed of a surface component and an internal, or volume component. The total reflectance (PT) may be given by: (2

where

p,

and

pi = internal reflectance (Leu, 1977).

=

-

21)

reflectance of the surface

Minerals show differences in the magnitudes of the surface and the internal components of reflectance depending on their refractive index and absorption. Most low absorptive minerals with low refractive index show a relatively small P

(1) and will become brighter due to a high

p

(A)

when grain size is

decreased. However high absorptive minerals are generally characterized by a high

p

(A)

and low

p

(A).

Most of the colours, we see around us are due to preferential absorption. The reflection from the surface is practically colourless; the principal colour is derived from light that has penetrated and which, after being reduced by the absorbing effect of the medium, is reflected by a second (or third) surface (Weiskopf, 1968). If the surface reflectance is very large, the material is said to have a surface colour (e.g.

metals).

When the radiation reflected from within the

material is predominant, the material exhibits a body colour which accounts for most minerals and vegetation. Diffraction occurs when EMR interacts with an edge oE an object. The reflected wave

fronts spread out

and

the EMR

departs from rectilinear

propagation. Scattering is closely connected both with reflection and with diffraction. When the size of a reflector is somewhat greater than the wavelength of the incident radiation, spherical and regular wavelets are produced in the centre

33 of the reflector to form short segments of plane wave fronts, while at the edges

the

reflected

wave

fronts

spread

out

owing

to

diffraction.

It

is

understandable, that there is greater spreading when the reflector becomes smaller with respect to the incident wavelength. The spreading may become so great that the reflected waves differ very little from uniform spherical waves, and plane wave fronts are not produced; the radiation is said to be scattered (Jenkins et al., 1957) Some of the earliest models assumed that scattering of the surface arose from many point scatterers and followed Lambert's cosine law:

I (Oi)

= I.

where I(Bi)

(2

cos Oi =

-

22)

intensity (W/sr) as a function of angle of incidence (0) and

I. = intensity (W/sr) for Bi = 0.

According to 2 0 , 6 4 I. and 0.34

-

22,

at,Oi l o " , 50" and 70", I (Bi) is respectively 0.98

Io,

10.

The distribution is hemispherical and is often referred to as the normal Lambertian behaviour. The model might be applicable for specific vegetation types when using coherent radiation for detection, but it does not explain the behaviour of scattering from many other surfaces. A

different approach is found in the so-called facet models. In these

models, a rough surface is described through a series of small planar facets. The models treat the scattering or reflection from the assemblage of facets by taking into account their size, slope, orientation and distribution. A wide finite facet, being many wavelengths across, shows a narrow reradiation

pattern, in contrast to a narrow finite facet (approaching the wavelength), which has a wider reradiation pattern. In Fig. 2.8 from left to right, the facet size is increasing. Small facets (at the left) are almost nondirective. Larger facets concentrate the scattered energy more at normal incidence. For an infinite large facet, all the energy is reflected back at the source.

When the illumination is incident from an other direction than normal to the surface, the general shape of the patterns remains the same, but the peak of the reradiation pattern is in the direction of reflection (Fig. 2.8b). Therefore, an infinite plane facet only produces much reradiation back at the

34

small

moderate

large

very large

moderate

(a) normal incidence Fig. 2.8

(b) oblique incidence

Facet patterns at normal (a) and oblique (b) incident illumination.

source when the direction of the incident radiation is normal to the surface. Natural surfaces, generally show a very complicated assemblage of facets, which may be described in a broad way by roughness. Rayleigh's criterion defines roughness as a function of wavelength and Oi. A surface is smooth according to this criterion if:

h < A/(8 cos Oi)

(2

-

23)

where h

=

height: above a plane in wavelengths. The

roughness

concerns

microrelief.

If

the

microrelief

becomes

significantly small, the surface is smooth and reflects with high directivity. When the microrelief is greater than the criterion given above, the surface scatters with a nondirective pattern. However, there will be an upper limit at which the influence of diffractive patterns is negligible and directive reflection dominates (see large facet Fig. 2.8).

As can be seen, the wavelength of the radiation dominates the roughness upon interaction. Most natural surfaces appear to be rough when illuminated by short wavelength radiation such as the Visible, but may be smooth for long wavelength radiation (Microwaves).

The angle of incidence is also important,

the smoothness criteria acc. Rayleigh being at 118 A s 1/5 A and '/3

Oi

= 10'

50' and 70' about

A respectively.

Another aspect related to the angle of incidence and surface roughness is

35 shadow. The shaded area has to be taken into account in calculating the total reflectance.

To illustrate this, Fig 2.9

is given, which shows the effect of normal and

oblique incident radiation on a surface with regular roughness. This figure demonstrates the complexity of roughness. Normal incidence in Fig. 2.9a

results in reflection at the horizontal surfaces only, their sum

being equal to the orthogonal projected surface. The total reflecting surface in Fig. 2.9b

consists of the horizontal upper facet as well as the vertical

facets towards the source and the horizontal lower facets minus the shaded areas of these facets. In this case the total reflecting surface is somewhat larger than the orthogonal projected surface. So

the effect of oblique incidence and consequent shadow is a small

increase in reflected radiation. The simple statement, roughness produces shadow and thus reduction in reflected radiation, has no general validity. The non- Lambertian behaviour of many natural surfaces, which may show a wide distribution of grain sizes, is mentioned above. The work of Coulson (1966), being a study on hemi-spherical reflectance of soil and grass surfa-

i nc ide nt r a d i a t i o n normal t o a surfa c e w i t h re gula r roughness

a.

\

b.

Fig. 2.9

i n ci d ent ra dia tion oblique t o a surfa c e with r egula r roughness

Incident radiation in relation to regular roughness of a surface of which each facet acts as a diffuse reflector.

36 ces, illustrates this (chapter 3 ) . Coulson found at high oblique incidence ( e.g.

Oi

=

53"), maxima in scattered

Visible and near Infrared radiation, not at the specular point, but forward beyond this point as well as in the backward direction (Fig. 2.10). and

pi (2

-

Using

ps

21) and the effect of shadow on rough surfaces, the following

explanation is given.

Fig.

2.10

Model of scattering from natural surfaces at high oblique illumination. The vectors indicate schematically the intensity of scattered radiation from a low absorbent medium.

At high oblique incidence of radiation on rough natural surfaces such as soils, the contribution of

ps

in the forward direction is reduced, since the

irregular surface minus the shaded area offers only few facets for reflection. However, for low absorbent materials,

pi, due to reflection from internal

facets, is high and contributes to the p in forward direction producing a maximum in the scattered radiation. In the antisource direction, however, no shadow is present to reduce Ps and another maximum is found. The latter will be more pronounced for high absorbent materials, which are characterized by a high p s

,

while the forward maximum for these materials is not pronounced,

owing to their low

pi.

2.7. Thermal properties A number of materials which show absorption due to electron orbital motion changes are capable of converting the absorbed energy into emitted radiation of a longer wavelength band without first converting the absorbed

37 energy

into

thermal

energy.

The

process

is

called

luminiscence

or

fluorescence. The fluorescence of natural materials is in the Ultraviolet, Visible or near Infrared. However, at the high frequencies of the Visible and the near Infrared, a considerable portion of the incident energy is wasted due to energy exchange with the components of the surrounding lattice. The absorbed energy ultimately appears as thermal radiation. In such a way, some of the solar radiation in the Visible is converted into middle and far Infrared upon interaction with the earth's surface. Some

concepts

and

parameters

used

in

description of

the

thermal

interaction process are discussed below (see also par. 2.2). The emissivity

E

is defined as the ratio of the spectral emissivity of a

material to that of a black body at the same temperature (2

-

5).

For a black

body, the spectral emissivity is equal to the spectral absorptance, which is by definition equal to 1. For natural bodies, we can distinguish absorbed energy ( M ), reflected energy ( MP) and transmitted energy ( M

). Therefore, the incident energy (Mi) can

be divided into (Janza, 1975):

+ M

M . = M i

a

+ M

p

~

2

-

24)

2

-

25)

or normalizing with Ei

a(X)+

p ( X ) +

r(X)=l

Natural bodies which approach the properties of black bodies with respect to emissivity are

the so-called opaque materials.

considered to have a

0 . Then a

+

p =

p

Opaque bodies may

1 and because a

is low and

E

be

= E:

(2

p

E = 1 -

where

T =

-

26)

is high for opaque natural bodies, while the opposite is

true for highly reflectant natural bodies. Similar to reflectance, one may use the terms absorptance and transmittance. Absorptance is defined as the ratio of the radiant flux absorbed hy a body to the radiant flux incident upon it. Spectral absorptance refers to the

38 absorptance in specified wavebands. Transmittance is the ratio of the radiant energy transmitted through a body to that incident upon it. The emissivity, like reflectance, is sensitive to variables of

look angle,

wavelength and polarization. Janza (1975) used an air-sand interface (a lossless medium with a complex dielectric consistent E~

=

3.2

to indicate the magnitudes of

-

j0 at a real temperature of 27510 for example

the parameters and their variations with the

angle of incidence. Since it concerns a lossless medium, the emissivities f o r vertical polarization and horizontal polarization, respectively ( are given by 1 E

h

/e

-

p

and 1 -

p

E

and

tj,

1,

h, respectively. In that case, the %/O and

curves can be constructed (see Fig. 2.11).

3=

w

1.0

2 0.9

0

," +J

0.8

a,

0.7

8

.r

V

E 0.6

x

c,

.r

0.4

5 .r

VI

0.3

v)

.-

5

0.2 0.1

0 0

Fig.

10

20

30

40 50 60 70 80 90 Angle o f incidence, 8 (degrees)

2.11

Emissivities f o r air-sand interface as a function of angle of incidence (Janza, 1975). (Used by permission of Am. S O C . for Photogrammetry and Remote Sensing.)

Lillesand et al., (1979) give typical emissivity values, the measurements being taken normal to the surface of the objects and at a temperature of 20°C.

39

By way of illustration some of these values are given below: Material

E

distilled water

0.96

wet soil

0.95

dry soil

0.92

sand

0.90

wood

0.90 The radiometric temperature (TB) of an object can be related to the real

temperature (T or thermometric temperature) by using the emissivity values )

(E

TB

=

therefore (Janza, 1975): ET

(2

-

27)

The third measure of temperature is the antenna temperature or TA, as it is measured from remote distances. This measure has to be calibrated against a

standard in the radiometer. In relating the calibrated TA with the TB of the object, the condition of the atmosphere between the object and the antenna has to be taken into account. To indicate temperature change or heat transfer in a medium or a system, a number of expressions are used. The specific heat ( C in Jkg-lK-')

of a substance is the ratio of the total

quantity of heat required to produce unit temperature change in a unit mass of that substance, to that required to produce the same change in a unit mass of water at 15" C (Janza, 1975) The thermal capacity or the ability of material to store heat is equal to the product of density (

p

in Kg. m-3)

and specific heat:

pC

in Jm-3K-'

(at

constant pressure; Reeves, 1975). The thermal conductivity A in JS-lm-lK-l or Wm-lK-'

is defined to be equal to

the quantity of heat that flows through a unit area of a plate of unit thickness, having unit temperature difference between its faces. It is a measure of the ease of heat transfer within a substance. Metals have high conductivity values, while those of insulating materials are low. The change in temperature caused by a certain quantity of heat flow is expressed by the thermal diffusivity (a in m2s-l),

which is equal to:

40

The thermal inertia is a measure of the rate of heat transfer at the interface between two dissimilar media. Materials with low thermal inertia are relatively insensitive to change in temperature at their boundaries. They feel cooler in the hand when hot, and warmer when cold, since the rate of heat transfer is low. The thermal inertia ( P in Jm-' P =

K-'

S M f ) is given by:

m

(2

-

29)

Some examples may serve to illustrate the differences and relationships between the parameters. Materials with a low density normally have a low thermal conductivity. Cork is extremely insensitive to heat flow, due to its high content of air. Air has an extremely low thermal conductivity. The thermal diffusivities of wood and brick may be the same (although wood has a low A and

pC, while brick has a high X and

pC),

that is the change in

temperature in these materials takes place within nearly the same period of time. However, the thermal conductivities are very different and cause the quantities of heat in these materials under the same temperature gradient to be very different. The difference may be expressed by their thermal inertia values. 2 . 8 . Atmospheric effect on EMR

The extraterrestrial solar radiation is influenced on its way to the earth's surface by absorption, scattering, direct reflection at clouds and refraction.

Spectra of the absorptivity of the main constituents of the

atmosphere, and the atmosphere as a whole are given in Fig. 2.12. The most important absorbers of radiation in the atmosphere are: oxygen, ozone, carbon dioxide and water vapour. However, the atmosphere is largely transparent between A 0.3

u m and

0.7

u m.

With increasing wavelength, more or less sharply defined absorption bands alternate with relatively transparent zones. These transparent zones, also known as "windows", are of great importance for remote sensing.

41

31

.r

>

.r

c,

a 0 & I VI

n

4

0 1

0 1

0 1

0 1

0 0.

Fig. 2.12

Spectra of absorptivity of the atmosphere and the atmosphere as a whole (modified after Barrett and Curtis, 1976; originally Fleagle and Businger, 1963).

Scattering by

atmospheric particles

alters

the direction of

solar

radiation in a random way.

Its impact is related to the wavelength of

radiation, the diameters of

the particles, and the optical density of the

atmosphere as well as its absorptivity. The three common types of scattering are (Barrett and Curtis, 1976):

-

Rayleigh scattering, which refers to interactions of solar radiation with molecules and other tiny particles with diameters much less than the incoming radiation;

-

Mie scattering, which is active in the presence of spherical particles

whose diameters approximate the wavelengths of the incoming radiation, e.g. small water droplets (slightly overcast sky) and dust particles;

-

Non selective scattering, which occurs when particles with diameters several times the wavelengths of the incoming radiation are present, e.g. large water droplets (clouds, fog).

Rayleigh scattering helps to explain the dominance of blue in a clear sky and

42

of orange and red at sunset. It is characterized by an inverse fourth power dependence on wavelength (preferential re-emission).

Blue light ( 4 7 0 nm) is

scattered about four times as much as red light (650 nm).

Clear skies,

therefore, show up in blue, owing to the strong scattering of short wavelength radiation. At sunset, the short wavelength is cut out mainly by powerful scattering due to the great pathlength through the atmosphere, and only the long wavelength radiation (including yellow, orange and red) reaches the earth's surface. In Mie scattering, the scattering properties of particles with identical absorption are determined by the R/X ratio (radius over wavelength ratio). The so-called Mie extinction coefficient u (extinction

OK

attenuation due to

scattering and absorption), is equal to the product of the number of particles (N)

times the extinction cross-section:

u= N K

(2

nR2

-

30)

where K = the extin tion fa

or, KnR2 = the extinction cross-section. The wavelength dependency in Mie scattering is different from that in Rayleigh scattering. In contrast to the latter, 'Mie scattering tends to influence longer wavelengths. Generally, both are active, and, depending on the particular atmospheric condition the colour of the atmosphere can range from blue to nearly white. Non-selective scattering causes all wavelengths of the Visible radiation to scatter with equal efficiency.

whitish, since a mixture of

As a consequence clouds and fog appear

all colours in approximately equal quantities

produces white light. Differences in intensity of solar radiation at the surface of the earth may be due to one or more of the following aspects (see Robinson, 1966 and Gibson, 1978) :

-

variations in the radiant emittance of the sun; variations in the distance between earth and sun; amount of water vapour in the atmosphere; dustiness of the atmosphere; altitude of the sun depending on latitude and time o f day;

43

-

elevation of the surface. The amount of water vapour and the dustiness of the atmosphere may be

expressed by the optical transmittance, while the altitude of the sun and the elevation of the surface are factors which determine the path length through the atmosphere. Yost and Wenderoth (1969) discuss the spectral distribution with time of day and with atmospheric conditions at Davis, California on 31 July 1967. From Fig. 2.13,

the wide variation i n spectral distribution which may occur

during a clear day is evident. There is a relative increase i n transmitted Infrared radiation at 750 nm in the afternoon (16.00

and 17.00)

as compared

with that i n the morning. This is thought to be caused by strong scattering of the Visible radiation by relatively large dust particles which have been churned up in the fields during the day.

m v ) Q L W a J

m c , I a J O

E

L

O

u .-

s m

E

C

C

\

'7

N

h L

m-.

140

1

t i m e o f day

- - 09.30

....... 09.55

120

u p p e r c u r v e 12.05

100

----I600

-1zoo

E U

80

s W

aJ

60 40 20

0 350

450

550

650

750

850

950

1050

1150

Wavelength i n nanometers Fig. 2.13.

Spectral distribution of incident solar radiation (during a clear day, 31 July 1967) at Davis, California, after Yost and Wenderoth (1969). Reprinted from 'Remote Sensing in Ecology', edited by Philip L. Johnson, 0 1969 the University of Georgia Press. Reprinted by permission o f the University of Georgia Press.

It will be evident that the spectral distribution with respect to the local conditions have to be taken into account in quantitative studies on remote sensing.

44

The spectral difference between direct

solar radiation and diffuse

skylight, which is the o n l y illuminant of shady areas, is illustrated in Fig. 2.14.

The shady areas are low-intensity regions with a spectrum that shows a

maximum in the blue region. Sensing in the blue may therefore reveal specific properties in these areas. V I V )

c , L

I

c , a J

/-.‘. ., \

\

c . ‘rNE

x u

I

60-

./ ‘\

I

L W S

0 S

\

I

m-.

c,

\

I

40

W -0 .r

u

-

S H

20

0

\

\ \

I I I I I

\ \ \

\

\

\

I

I I

I

\

,’--, \- 1

-

I

1

I

1

1

I

\

\ \ \ \

\

>

I

45

Furthermore, the high penetration capability of Microwaves for rain is evident from the data given in Fig. 2.16, e.g. and X

=

microwaves with X

=

3 cm at heavy rain

6 cm at excessive rain, only show 0 , 5 dB/km attenuation. This helps to

explain the enormous advantages of the application of aequatorial areas with tropical rainforest. Microwaves

4

--

radar in the wet

Infrared

-

wavelength (m)

1 cm

10 cm

1 mm

100 pm

10 pm

500 200 100 50

20 10 5 2

1

0.5

0.2 0.1

0.05 0.02 0.01

2 3 5

10

1o 2

lo5 Frequency ( GHz)

Fig. 2.15

2.9

Atmospheric attenuation (dBkm-l) for a horizonta path at a temperature of 293 K and a watercontent of 7.5 gm-' (Krul, 1982; derived from Preissner, 1978); dB = 10 log Pl/P2, where P1 = power top atmosphere and P2 = power after passage of 1 km atmosphere.

Energy balance With the information given in the previous sections, a two-dimensional

model on the interaction of solar radiation with the earth's surface can be composed; such a model is given schematically in Fig. 2.17.

-

46

Microwaves

4

-

Infrared

Wavelength (m) 1 cm

1 0 cm

L 0

r

m L 0 + h

E

. .. m

100 pm

1 mm

10 pm

50 20 10 5

Y

-0

v

K

2

1

0 .,-

c,

m

3 K

aJ

c, c,

m

u

+ .C

.r

V a W

wl

2 3 5

10

1o3

1 o2

lo5 Frequency (GHz)

Fig. 2.16

Atmospheric attenuation for different rain intensities and for f w or drizzle conditions ( K r u l , 1982; derived from Preissner, 1978).

On its way down to the earth's surface, the incoming solar radiation is altered by the various processes active in the atmosphere. The radiation which reaches the earth's surface under clear weather conditions, is composed of a direct and a diffuse component. Interaction with objects at the earth's surface causes

the

incident

radiation

to

be

reflected,

refracted,

absorbed

or

transmitted. The absorbed energy can be reradiated (emission). The interaction process is guided by the following principle: the energy sum of the different components active in the interaction is equal to the sum of the incident energy. However, over a certain period of time, there can be a gain in radiation upon interaction, the so-called net radiation which is composed of the following components (Janza, 1975):

(k in

wm-2),

47

J

transmission

Fig. 2.17 Two dimensional interaction model of solar radiation with an object at the earth’s surface. R,

=

where

Ris

+

Ris

Rid

+

Rit

-

[ P (Ris

+ Rid) + Rot 1

= incident direct solar radiation (Wm-’);

Rid

=

incident diffuse solar radiation (Wm-’);

Rit

=

incoming longer wavelength radiation (Wm-’) ;

Rot

=

outgoing longer wavelength radiation (Wm-*) ;

p

= reflectance of the surface.

(2

-

31)

48 The net radiation is used for a number of processes (Janza, 1975): S

Q =

+

A

+

LE

+

P

+M

( 2 - 32)

where S

=

A =

heat radiation from or into the soil ( ~ J I I I - ~ ) , heat radiation from or into the air (Clm-2 ),

LE = radiation used for evapotranspiration (Ilm-2), P = radiation required for photosynthesis (Wm-2), M

=

radiation required for miscellaneous conversions (Wm-2).

2.10.

Spectral reflectance By way of introduction to chapter 3, spectral reflectance is treated

below by giving a summary of spectral features. The spectral reflectance is the ratio of the radiant energy within a specific wavelength range reflected by a body, to the incident energy within the same wavelength range. Spectral reflectance curves may be used to indicate spectral properties. The curves may be characterized by features like absorption maxima, denoted below as bands. A summary of these features in the 0.4-2.5 given in Fig. 2.18.

um wavelength region is

The bands found in this spectral range are related to the

presence of H20, Fe(II),

Fe(III),

OH'

OK C03"

in solid matter (see Fitzgerald,

1974). The bands generated by electronic processes (see par. matter are generally broad and

OCCUK

2.4)

in solid

in the Ultraviolet, extending less

frequently into the Visible and near Infrared with a band at 1.1 m as a limit. The ground state of ferrous iron in an octahedral electrical field splits into two levels. The transition allowed between these levels gives rise to the band at 1.1 urn. Transitions between the remaining levels of ferrous iron do only result in very weak bands at 0.43 urn, additional

band

can be

0.45 urn, 0.51 !m and 0.55 um. Furthermore, an

produced

at

1.8-1.9

urn

in a

highly

disordered

octahedral site. Ferric ion has a ground state that will not split in any crystal field. Transitions to higher levels appear only weakly in the spectrum, e.g.

at

0.4 urn and at 0.7 urn. Other transitions 3re observed at 0.45 m, 0.49 m and 0.87 urn.

Fy

49

..... .. ..... ..

0.4 pm

I

1

I

I

I

1

I

1 .o

0.8

0.6

- o f C O i

OH ' Fp'

Fe"'

FQ

!!! I! ! H-0

HO ,

I'

I'

I . . +

O H ' OH' 1

1

L

A 9

1.0

pm

1.5

strong

Fig. 2.18.

1

weak

I

1

'

2.5

2.0

b r o a d band

I

s h a r p band

Absorption bands due to electronic and vibrational processes in the - 2.5 urn wavelength range of the EMS.

0.4

The bands produced by vibrational processes in solid matter are relatively sharp. The vibrational features observed in reflectance spectra in the Visible and near Infrared are due to overtones or combination tones of H 2 0 , C03".

OH'

and

The fundamental vibrational bands can be found in the mid- and far

Infrared. Overtones occur when a fundamental mode is excited with two or more quanta. Combinations occur when two or more fundamental modes and/or overtones of different modes are added or subtracted. Water molecules may occur at various sites in minerals:

-

as free molecules in small interstices or pockets (e.g.

in quartz);

- singly or in clusters as a part of the crystal lattice (e.g. gypsum); - in specific sites in the crystal lattice without being essential to its structure (e.g.

in zeolites);

- physically adsorbed at the surface of mineral grains and between the layers of layer-silicates. The variety in sites also leads to a variety in frequencies of the fundamental modes. In the near Infrared, two water absorption bands occur at 1.4 urn and

1.9 um, respectively, due to overtones or combination tones of the water fundamental. When these bands are sharp, the water molecules are supposed to be located in well defined ordered sites. Broad bands indicate the water

50

molecules to be relatively unordered and at various sites. the hydroxylgroup, the OH stretching mode, results in

The vibration of bands at 1.4 um and 2.8

vm. Combination of the OH stretching mode (at 1.4 m)

with lattice vibrational modes, produces a band at 2.2 Itm. Layer-silicates oriented.

and

Variation

micas in

the

show OH-groups orientation of

which

are

strongly direction-

the OH-groups, due

to

Si-A1

substitution, produces a broadening of the band at 1.4 Lim. I ,

Furthermore, overtone and combination tones of internal vibrations of C03 anion radical, or with the lattice vibrations, result in bands between 1.6

~m

and 2.5 Lim. Data on spectral features covering most of the Infrared region are given by Kahle et al., (1980) and Siegal et al., (1980). A summary is presented in table 2 . 3 .

Interesting is the possibility of detecting gypsum by using near

Infrared radiation. Silicates show

intense absorption

due

to

the

silicon-oxygen

Stretching

vibration at 10 um. However, at the onset of this absorption band at 7-9

m,

they show a peak in reflection. Below, some attention is paid to an important constituent of soils: organic matter. Schmitzer et al.,

(1972) and Flaig et al.,

(1975),

present

data on the spectral reflectance of soil organic matter. Soil organic matter is composed o f : a)

nonhumic substances such as carbohydrates, proteins, peptides, aminoacids, fats, waxes, resins and pigments;

b)

humic substances being humic acids, fulvic acids and humins.

The assignment of specific absorption bands is limited by the fact that in most cases, soil organic matter consists of mixtures of complex molecules, and therefore shows overlapping of absorption bands. To get an impression of its complexity, the main Infrared absorption bands of humic acids are given in table 2.4.

In addition to the modes

presented in this table OH and Si-0-Si are frequently found in soil organic matter. The utility of the bands in table 2.4 has to be tested for remote sensing. 2.11. Conclusions EMR may be generated by a change in electronic energy levels and by

changes in the vibrational and rotational energy of atoms, ions and molecules. It occurs in wave trains or bursts of radiation that carry a radiant energy

51 Table 2.3 Summary on vibrational features according to Siegal et al., (1980). bands in m

Constituents modes symmetric stretch asymmetric stretch H-0-H bend stretching fundamental Al- OK Mg - OH bend Al - OH bend fundamental Fe-0 fundamental stretching

H2O OH' oxides hematite carbonates phosphates sulphates gypsum

overtones and combination of OH stretching in molecular water fundamental bending mode of constitutional water Si-0 bending Si-0 stretching H-0-A1 bending Si-0-Si, A1-O-Si stretches Si) stretch (Si, A l ) - O - ( A l , deformation and bending modes of 0 - ( A l , Si)-0, (Si, Al)-0-(Si, Al), O--(Al. Si)-O.~ Al, Si-0-metal valence stretching

silicates

Table 2.4

3.106 2.903 6.08 2.77 2.2 OK 2.3 11 5 20 7, 11-12, 13-15 9.25, 10.3, 18, 28.5 9, 10.2, 16, 22.2 1.75,

2.3

6

around 5 10 11 12-15 15-20

20-40 20-40

Main Infrared absorption bands of humic acids after Flaig et al., (1975).

modes

-

bands in um

C-H C-H, C-H2, C-H3 carboxylate ion

3.25-3.30 3.39-3.50 3.50-4 .00 5.80-6.10 6.10-6.3 1 6.50 6.60 6.80-7.0 5 7.20-7.50 7.80-8.80

c=0 c=c NO C=Z C-H deformation salts of carboxylic acids

co

which is proportional to the frequency and inversely proportional to the wavelength.

Two laws of radiation for black bodies are formulated: 1)

the total of

radiation emitted from a black body is proportional to the fourth power of its

52

absolute temperature; 2) the wavelength of the maximum radiant emittance of a black body is inversely proportional to its absolute temperature. There is a spectrum of

EMR with wavelength regions such as Ultraviolet,

Visible, Infrared and Microwaves. The particular zones are essential for life (Visible and Infrared) or are made use of for practical reasons (Microwaves and Radiowaves). Solar irradiance has its maximum at approx 0.5 um. Terrestrial emittance shows a maximum which is located at approx 10 um, and has a very low energy level as compared to solar irradiance. The atmosphere modifies solar radiation by absorption

and

scattering

before

it

reaches

the

earth's

surface.

The

absorption by atmospheric particles is relatively strong in the Infrared, but some wavelength zones are relatively free of absorption. These are known as windows and are of much importance to remote sensing. Spectral reflectance may reveal specific properties of materials at the earth's surface. However, the atmospheric windows determine its potential use in remote sensing.

2.12.References Barrett, E.C. and Curtis, L.F., 1976. Introduction to Environmental Remote Sensing. London, Chapman and Hall: 336 pp. Coulson, K.L., 1966. Effects of Reflection Properties of Natural Surfaces in Aerial Reconnaissance. Applied Optics, Vol 5, No 6: pp. 905-917. Dekker, A.J., 1958. Solid State Physics. London, MacMillan h Co Ltd: 540 pp. Fitzgerald, E., 1974. Multispectral Scanning Systems and their Potential Application to Earth-Resources Surveys. Spectral Properties of Materials. ESRO CR-232, Neuilly, France: 231 pp. Flaig, W., Beutelspacher, H. and Rietz, E., 1975. Chemical Composition and Physical Properties of Humic Substances in Soil Components V o l . I. Organic Components (ed. Gieseking, J.E.). Springer Verlag, New York: 213 PP. Fleagle, R.G. and Rusinger, J.A., 1963. An Introduction to Atmospheric Physics. Academic Press, New York. Gibson, H.L., 1978. Photography by Infrared. Its Principles and Applications. John Wiley h S o n s , New York: 545 pp. Higham, A.D., Wilkinson, B. and Kahn, D., 1973. Multispectral Scanning Systems and their Potential Application to Earth-Resources Surveys. Basic Physics & Technology. European Space Research Organisation: 186 pp. Jamieson et al., 1963. Infrared Physics and Engineering. McGraw Hill. Janza, F.J., 1975. Interaction Mechanisms. Chapter 4 in Manual of Remote Sensing. Amer. SOC. of Photogrammetry, Falls Church, Virginia: pp. 75179. Jenkins, F.A. and White, H.E., 1957. Fundamentals of Optics. McGraw-Hill Book Cy, Inc., New York: 637 pp. Jordan, E.C., Balmain, K.G., 1968. Electromagnetic waves and Radiating

53 Systems. P r e n t i c e - H a l l I n c . , New J e r s e y : 753 pp. Kahle, A.B. and Rowan, L.G., 1980. E v a l u a t i o n of M u l t i s p e c t r a l Middle I n f r a r e d A i r c r a f t images f o r L i t h o l o g i c Mapping i n t h e East T i n t i c Mountains, Utah. Geology. The Geol. SOC. of Amer., B o u l d e r , Colorado: pp. 234-239. K r u l , L., 1982. F y s i s c h e Aspecten van d e A a r d o b s e r v a t i e w a a r b i j d e nadruk l i g t op h e t systeem. A g r i c u l t u r a l U n i v e r s i t y , Wageningen, The N e t h e r l a n d s . PAO-cursus " T e l e d e t e c t i e i n landbouw e n n a t u u r b e h e e r " : 1 1 pp. Leu, D . J . , 1977. V i s i b l e and Near I n f r a r e d R e f l e c t a n c e of Beach Sands: A s t u d y on t h e S p e c t r a l R e f l e c t a n c e / G r a i n S i z e R e l a t i o n s h i p . Remote S e n s i n g of Environment 6, E l s e v i e r N-Holland: pp. 169-182. L i l l e s a n d , T.M. and K i e f e r , R.W., 1979. Remote S e n s i n g and Image I n t e r p r e t a t i o n . John Wiley & Sons, New York: 612 pp. 1976. Remote S e n s i n g of Environment, AddisonL i n t z , J.Jr. and S i m o n e t t , D.S., Wesley Publ. Cy., Reading, M a s s a c h u s e t t s : 694 pp. P r e i s s n e r , J., 1978. The I n f l u e n c e of t h e Atmosphere on P a s s i v e R a d i o m e t r i c Measurements. AGARD C o n f e r e n c e Proc. No 245: pp. 48.1 - 48.14. P r i n s , J.A., 1955. G r o n d b e g i n s e l e n van d e hedendaagse Natuurkunde. \ J o l t e r s , Groningen, Nederland: 320 pp. R e e v e s , R.G., 1975. G l o s s a r y i n Manual of Remote S e n s i n g Vol. 11.. Amer. SOC. of Photogrammetry, F a l l s Church, V i r g i n i a ; pp. 2061-2210. Robinson, N. ( e d . ) , 1966. S o l a r R a d i a t i o n . E l s e v i e r Publ. Cy., Amsterdam: 347 pp. Schawlow, A.L., 1968. Laser L i g h t . S c i e n t i f i c American. Vol. 219, n r . 3: pp.

120-136. 1972. Humic S u b s t a n c e s i n t h e Environment. S c h m i t z e r , M. and Khan, S.U., Marcel Dekker, New York: 327 pp. 1980. Grootheden e n Eenheden i n de Landbouw e n S c h u r e r , K. and Rigg, J . C . , B i o l o g i e . Pudoc, Wageningen, The N e t h e r l a n d s : 121 pp. S e l l e r s , W.D., 1965. P h y s i c a l C l i m a t o l o g y . Univ. of Chicago P r e s s , Chicago. S i e g a l , B.S. and G i l l e s p i e , A.K., 1980. Remote S e n s i n g i n Geology. John Wiley & Sons, N e w York: 702 pp. Ulaby, F.T., Moore, R.K., Fung, A.K., 1981-1982. Microwave Remote S e n s i n g Vol. I and I1 Addison-Wesley P u b l . Cy., London: 1064 pp. Weast, R.C. (ed.), 1974. Handbook of Chemistry and P h y s i c s . CRC P r e s s , C l e v e l a n d , Ohio. W e i s s k o p f , V.F., 1968. How l i g h t i n t e r a c t s w i t h Matter. S c i e n t i f i c American, Vol. 219, n r . 3: pp. 60-71. Y o s t , E. and Wenderoth, S., 1969. E c o l o g i c a l A p p l i c a t i o n s of M u l t i s p e c t r a l C o l o r Aerial Photography. In: Remote S e n s i n g i n Ecology e d . by P.L. Johnson, Athens, Univ. of G e o r g i a P r e s s : pp. 46-62. 2.13.Additional

reading

1964. Thermodynamics. Pergamon P r e s s , London: 287 pp. Bazarov, I.P., R.I.P.M., 1977. The I n t e r n a t i o n a l System of U n i t s (S.1.). 3rd edn. H.M.S.O. London. I.S.B.N. 0-11-480045-6: 54 pp. C o l w e l l , R.N., 1963. Report of Subcomm. I. B a s i c M a t t e r and Energy R e l a t i o n s h i p s I n v o l v e d i n Remote S e n s i n g Reconnaissance. American S o c i e t y of Photogrammetry: pp. 761-809. D i t c h b u r n , R.W., 1976. L i g h t . 3rd edn. Vol. 1 and 2. Academic P r e s s , London 775 pp. Feynman, R.P., L e i g h t o n , R.B. and Sands, M., 1970. The Feynman L e c t u r e s on P h y s i c s . Mainly Mechanics, R a d i a t i o n and Heat. 5 t h edn. Adison Wesky Publ. Cy, Menlo P a r k , C a l i f o r n i a .

54

Heel, A.C.S. van and Velzel, C.H.F., 1967. Wat is licht? Wereldakademie, W. de Haan/J.M. Meulenhof: 245 pp. Goody, R.M., 1964. Atmospheric Radiation. Oxford, Clarendon Press. Holz, R.K. (ed.), 1973. The Surveillant Science. Remote Sensing of the Environment. Houghton Mifflin Cy, Boston: 391 pp. Kronig, R. (red.), 1962. Leerboek der Natuurkunde. Scheltema & Holkema N.V., Amsterdam: 891 pp. Longhurst, R.S., 1962. Geometrical and Physical Optics. Longmans, Green and Co. Ltd London: 551 pp. LOOK, G.P., de, 1983. The Dieletric Properties of Wet Materials. IEEE Trans. on Geoscce and Remote Sensing, Vol. GE-21, No. 3: pp. 364-369. Meyer-Arendt, J.R., 1972. Introduction to Classical and Modern Optics. Prentice-Hall International Inc., London: 558 pp. Monteith, J.L., 1973. Principles of Environmental Physics. Edward Arnold (publ.) Ltd, London: 241 pp. Peake, W.H. and Oliver, T.L., 1971. The Response of Terrestrial Surfaces at Microwave Frequences. Ohio State Univ. Electroscience Lab., Tech Rep. AFAL-TR-70- 301. Reeves, R.G. (ed.), 1975. Manual of Remote Sensing. American Society of Photogrammetry. Falls Church, Virginia. Vol. I: 867 pp. 1974. Remote Sensing. A better View. Duxbury Press, North Rudd, R.D., Scituate, Masachusetts: 135 pp. Wade, F.A. and Mattox, R.B., 1960. Elements of CKystallOgKaphy and Mineralogy. Harper & Brothers Publ., New York: 332 pp. Wahlstrom, E.E., 1954. Optical Crystallography. 2nd edn., New York, John Wiley & Sons Inc.: 247 pp. White, D.C.S., 1974. Biological Physics. Halsted Press. New York: 293 pp.

55

DATA ON INTERACTION OF SHORT WAVE ELECTROMAGNETIC RADIATION WITH NATURAL

3.

OBJECTS.

In

this

chapter,

emphasis

is

given

to

the

results

of

laboratory

measurements on reflectance and thermal parameters. The ranges of the EMS covered are those between 0.4-2.5

urn and

8-14

um

, which form important

portions of the spectra of solar irradiance and earth emittance. In par. 3.1,

minerals as constituents of rocks and soils are discussed. Later

on in par. 3.2,

being

the reflectance and thermal properties of soils are dealt with,

highly

influenced

by

moisture

condition,

organic

matter

content,

structure and texture. A summary on the properties of plants and plant canopies is given in par. 3.3. Laboratory

measurements

combinations

of

these

normally

concentrate on

representing part

of

the

individual components

or

natural variation.

The

laboratory details have to be transformed to the assemblage of components as depicted by remote sensing aids (par.

3.4).

This chapter, therefore, is a

transition between interaction theories (chapter 2)

and remote sensing data

such as given in the chapters 9-11. 3.1.

Interaction of short wave radiation with minerals and rocks.

sF!ectELrerlect2_nce Minerals occur in cemented granular form in duricrusts and hard rocks or in loose granular form in unconsolidated sediments, rotten rock and soils. A comprehensive text on the Visible and Near Infrared spectral features of minerals is given by Hunt, Salisbury et al., (1970-1976).

A summary of part of

their work is provided by Fitzgerald (1974). Below a brief treatise is given on the spectral properties in the Visible and Near Infrared of dominant minerals such as:

-

quartz and feldspar; amphibole and pyroxene; mica and layer-silicates; limonite; carbonate and gypsum. Quartz shows a very high reflectance and the spectrum in the Visible and

near Infrared

is almost devoid of spectral features (such as absorption maxima

denoted as bands), unless impurities occur. The same is true for feldspar.

56

Only, water-bearing fluid inclusions and contamination by iron result in spectral features. Amphibole shows a band near 1 urn,

indicating that it contains ferrous

ions. It displays a very sharp hydroxyl band at 1.4 um and less sharp bands between 2.0 um and 2.5 urn. The latter are due to overtone and combination tones of

the OH stretch. The bands mentioned are characteristic for the variety

Tremolite. Another variety, Actinolite, shows in addition a broad band near 0.7

wn due to

the presence of ferric ions (see fig. 3.1.a). Pyroxenes do not show the hydroxyl bands unless some alteration has taken place. The relatively high content of

iron is expressed by a broad band at

0.9 um, and in the variety Hypersthene by an additional broad band at 1.8

rn

which is probably due to ferrous ions in a highly disordered octahedral site (see fig. 3.1.b). Amphibole, v a r . A c t i n o l i t e

Pyroxene, v a r . Hypersthene

8

v

a, S V

rn +

50-

7

le E a,

0

,

I

I

1

I

A Fig.

3.1

I

I

I

h

A i n pm

i n pm

(a) (b) Reflectance (relative to MgO) of Amphibole, var. Actinolite, and Pyroxene, var. Hypersthene; grain size 74-250 prn (after Hunt and Salisbury, 1970).

The mica var. Muscovite displays hydroxyl bands at 1.4 pm as well as between 2.2

um and 2.6 um.

Biotite may show a much broader hydroxyl band and in

addition shows a very broad band in the 0.6

to 1.5 urn region, due to ferrous

and ferric ions. Layer-silicates are characterized by hydroxyl bands centered near 1.4

~rm

and 2.2 D m. Absence of appreciable amounts of bound water is typical for Kaolinite. Therefore, it shows only a weak band at 1.9 u m (fig. 3.2.a).

On

the contrary

57

Montmorillonite is capable of holding much water and may show very strong bands at 1.9 um as well as at 1.4 u m (see fig. 3.2.b). also a waterband, where as the bands at 0.9

u m is u m are due to the

The band near 1.15

u m and 0.5

Kaolinite

Montmorilloni te

1

0.5

1.0

1.5

2.0

2.5

1

I

I

0.5

1.0

1.5

I

2.0

A in p m

Fig. 3.2

I

I

2.5

A in prr

Reflectance (a) (relative to MgO) of Kaolinite ( b ) and Montmorillonite; resp. with grainsize 0.1-5 u m and 0.1-74 u m (after Hunt and Salisbury, 1970)

presence of ferrous ions. Limonite is often used to indicate hydrated ferric oxide material while goethite is synonymous for the ferric oxide hematite. The water content of limonite is variable. The band at 0.9

u m typical of the ferric oxides and

hydration bands near 1.4 u m and 1.9 u m may show up in the spectra. Calcite exhibits carbonate bands between 1.8 u m and 2.5 display a very weak hand of ferric ion

at

0.8

LI

m, and may

u m. The latter may be due to

iron contamination in often very small amounts. Fine grained calcite has a very high reflectance as is shown in fig. 3.3. 2.35

Note the strong absorption band at

u m.

Gypsum shows bands at 1.8 LI m and at 2.3

LI

m, due to overtones and combination

of OH stretching in molecular water. If we consider rocks, we mostly have to deal with an assemblage of minerals, and various spectral features will occur e.g.

limestones may display

carbonate bands as well as water and hydroxyl features, being dependent on the admixture of constituents such as layer-silicates. Features due to the presence of ferrous ions (intrinsic to the presence of clay) and ferric ions (coatings)

58 may also

OCCUK.

emissivity -Spectral ----------------The spectra of emitted thermal radiation from minerals are generally characterized by emissivity minima in the 7-15

!J

m region. These minima are

known as the "reststrahlen" bands and are due to vibrational transitions of the

m

c,

u W

+ 7

Fig.

50-

Reflectance (relative to MgO) of Calcite of grain size 74-250 m (after Hunt and Salisbury, 1971).

3.3

main anion of the mineral.

In silicates, the fundamental frequency of the Si-0 stretching mode occurs near 10

!J

m and the 0-Si-0 bending or deformed ion mode is found near 20 IJ m.

The fundamental Si-0 vibration near 10 rock.

!J

m shifts with the type of igneous

Quartz shows an emissivity minimum at 9

!J

m, which is the shortest

wavelength of any emissivity minimum of silicates. The spectral emissivity of a number of igneous rocks is given in fig. 3.4.

From

this figure, we can read that the examples given of acid-igneous rocks have emissivity minima from 8.8 to 9.6

u m (e.g.

granite near 8.80 p m), while

those of basic and ultrabasic igneous rocks have emissivity minima higher than 10.1 !Jm (e.g.

Limburgite 10.53 IJ m).

The intermediate igneous rocks show minima

between 9.2 and 10.0 u m. To discriminate silicates from non-silicates, the spectral emissivity may be used.

For example, carbonates show, much contrast with silicates in having

strong absorption at 7.0

iI

m.

The diurnal variation in surface temperature changes of rock formations is

59 the most significant short term variation in rock properties that can be used in remote sensing. The difference in the amplitude of the diurnal variation in temperature between rocks is due to their differences in thermal inertia. The thermal inertia (2-29)

is a function of the thermal capacity and conductivity

as influenced by porosity, texture, structure, chemical composition and

>

c, .I-

>

Andesi t e Nepheline Syenite

.I-

In In .I-

E

W

INTERMEDIATE ROCKS

7

m L

Hypersthene Andesi t e

c,

u

~

W

n

Quartz D i o r i t e Augite D i o r i t e

Ln

L 1.1

1

D i or it e - - --e Aug - -it -

I

BASIC ROCKS

Basalt

- - - - - - - - -1- -

%

Perid o t i t e Serpentine

___-------I

Fig. 3 . 4

1

1

8

9

I

I

,

1

,

10 11 1 2 1 3 pm

Spectral emissivities of Lyon, 1965)

i

ULTRABASIC ROCKS Limburg it e

10.53

igneous rocks from 8-13 m (modified from

60 moisture content. The day-night temperature difference can be used to calculate the thermal inertia of surface materials. 3.2

Interaction of short wave radiation with soils. The interaction of solar radiation with soils plays an important part in

soil forming processes and more specific in the heat balance of soil. The dry/wet soil colour designations are actually observations on spectral reflectance, which may indicate the presence of organic matter and of oxidized OK

reduced iron compounds. Although

soils are composed of granular mineral materials which are

generally mixed in their surface layers with organic matter, their reflectant and emittant properties are greatly influenced by moisture condition, texture and structural arrangement of the constituents which often predominate over the effects of chemical composition. Spectral reflectance _____-------__-----Several authors have provided spectral reflectance data of soil materials which have been obtained under laboratory conditions (Orlov, 1966; 1969-1970;

Skidmore

et

al.,

1975;

Gold

and Asher,

1976).

Planet,

I will first

concentrate on spectral features which are due to chemical soil composition: Obukhov and Orlov (1964) present some curves in the spectral zone from 0.40 to 0.75

um, of which the results are given in fig. 3.5. Three types of curves are distinguished in this wavelength range:

a) monotonously rising curves, from short to longer wavelengths; the slope of the curves becomes somewhat weaker at the longer wavelength end (fig. 3.5 nrs 1, 3 and 6);

b) a curve with minor slope and low reflectance values (fig. 3.5

nr. 4 ) ; the

low reflectance values apparently are due to the high content of organic matter; c) the type of curve represented by nr. 5 in fig. 3.5; increases at a moderate rate up to about 0.53

LI

the slope of the curve

m and then rises sharply to

about 0.58 u m, where a definite decrease of slope occurs; this type of curve is typical for samples rich in ferric ion, which show absorptance in the shorter wavelength range and high reflection in the orange and red. For further discussion on types of curves, one is referred also to Condit (1972),

who gives special reference to American soils.

61

3.5

Fig.

Spectral reflectance data of Russian soils (modified sketch after Obukhov and Orlov, 1964). 1. 2. 3.

4.

5. 6.

* Valuable

Sod-podzolic soil, A1 0-10 cm; OM* 1.6 %; clay loam. Grey Forest soil, A1 15-26 cm; OM* 3.8 %; silty clay loam. Light Chestnut soil, A1 0-10 cm; OM* 2.7 %; clay loam. Chernozem, Asod 0-5 cm; OM* 10.3 %; silty clay loam. Red coloured soil on limestone, A1 4-11 cm; OM* 3.2 %; clay. Light coloured Sierozem, A1 0-10 cm; OM* 1.1 %; clay loam. OM or organic matter content.

information about

the composition of

soils may

be

obtained by

extending our view into the near Infrared. Soil reflectance spectra including the near Infrared (as well as the Visible) are reported by several authors: Bowers and Hanks (1965), (1975),

Mathews et al., (1973),

Janse and Bunnik (1974), Damen

Janse et al., (1976) and Girard (1977).

Numerous reflectance measurements of American soils in the 0.52-2.32 wavelength range have been presented by Stoner et al.,

(1980).

pm

It has been

pointed out that soils rich in organic matter (Histosols, Mollisols) frequently have a concave shaped reflectance curve between 0.5 soils low in organic matter (e.g.

Alfisols)

P m and 1.3

P m, whereas

frequently show convex shaped

curves over the same wavelength region. Ultisols often resemble the curves of Alfisols but they additionally show weak absorption bands at 0.7 0.9

P

and near

m caused by the presence of iron.

Besides the chemical composition, other soil properties such as moisture content, texture, structure and roughness of the soil surface have a marked influence on reflectance and thus have to be evaluated. The influence of soil moisture content on the reflectance of silt-loam as

measured by Bowers and Hanks (1965) is often cited in literature. I present

62

the curves in fig.

3.6.

The water absorption bands (1.4

clearly marked as well as a weak hydroxyl band (2.2 p m).

and 1.9

u m) are

Increase of soil

moisture content results in an overall decrease of reflectance in the Visible as well as in the near Infrared. Damen (1975)

states that soil moisture tension values are most suitable

for analysis of soil moisture, and presents spectral reflectance curves of soil material at different soil moisture tensions. The water absorption bands are clearly marked in the range of soil moisture tension form 0.005 bar up to 16 bar and the total reflectance, as can be expected, decreases with decrease of h

3

60

a, V

m

+J u

a,

G 40

20

0

V

I

U I

Fig.

3.6

1 obo

t h (nm) 2000 Wavelength

Spectral reflectance of Newtonia silt-loam at various moisture contents (moisture contents indicated directly above each curve) after Bowers and Hanks (1965).

soil moisture tension (fig. 3.7). the

I

macropores

of

soil

At low soil moisture tension, water fills up

(gravitational water)

and

strongly

reduces

the

reflectance. Cohesion and adhesion water, which can be held at high moisture tension in resp. micropores and at the surfaces of soil particles, have considerably less influence. In fig. 3.6

it is striking that the dips at 1.4 u m and 1.9

u m become deeper

and broader with increasing moisture content. Relatively sharp absorption bands are

characteristic of

low moisture contents and thus high soil moisture

tension. Another remark may complete the discussion about these interesting curves. The hydroxyl band at 2.2 um is most clear at low moisture contents (fig. 3.6);

at

high moisture contents, it becomes vague. Therefore, its use in the detection of layer-silicates may be restricted to relatively dry soils. Texture of soils has got attention in soil reflectance measurements, too.

63 h

d?

SMT

-

v

E60-

16

c

c m ,

u

a,

3,2 ....... ........... 0,5 / - - -

-

0,032

7

%

bar " " "

Fig. 3 . 7 Spectral reflectance in relation to soil moisture tension of a soil with a clay content of 9 . 3 X (after Damen, 1975).

Generally, fine textures show a higher reflectance than coarse textures. Leu (1977)

reports that the spectral response in the 0.43-0.47

!.I

m and 0 . 5 1 - 0 . 5 3 ~

channels is correlated with the grain size of beach sands having variable moisture contents. A means to determine the average size of particles may be found through application of high oblique illumination. The ratio of intenties at a forward angle to that at a back angle can be used for this purpose. For the same material (e.g.

quartz sand), the scattered light becomes more concentrated in the

forward direction with increasing particle size (Meijer-Arendt, 1972). Besides grain size, the reflectance of soils will also be influenced by aspects such as sphericity, roundness (Brewer, 1964) and the micro-roughness at the surface of the grains. Several authors report about

results of

reflectance measurements in

relation to aggregate size. Orlov ( 1 9 6 6 ) studied various soil samples with a range of aggregate diameters. In general, he found for small sized aggregates a decrease in reflectance with increasing diameter. Huwever, at large diameters (>

2.5 mm) there was only a slight or no decrease in reflectance.

Damen (1975) has also studied the influence of aggregate size on reflectance. Reflectance values of a loamy topsoil sample of Woudgrond with different

64

aggregate sizes are given in fig. 3.8. Furthermore, he studied soil roughness by creating fine and coarse rills on the surfaces of samples (fig. 3 . 9 )

and found the coarse rill pattern to show the

lowest reflectance.

It is evident, therefore, that the structural condition of the soil surface is of great influence on reflectance e.g.

soil surface crusts may cause high

reflectance values in the Visible (Cipra et al., 1971). Some authors report about the variation of reflectance with the angle at which the radiation is incident on the surface. Coulson (1966) gives a summary of previous research and presents results on directional reflectance of different mineral materials. Some of the curves are shown in fig. 3.10 (angle of incidence 0

-2 -Woudgrond 860S

cr u 1

.

u

............ .......

c

5 3 " ) . The reflectance is measured

( a i r dry)

0.3 m n ~

0.3-1 1 -2

Fig. 3.8

=

" "

Spectral reflectance at different aggregate diameters of Woudgrond (with 7.4 % C and 12.0 % clay) after Damen (1975).

hemispherically in the principal plane, that is the plane containing the source, the object and the measuring device. Materials with a low absorption like gypsum sand and beach sand (quartz) show a high reflectance and a strong forward maximum in scattering. Absorbent materials like clay, limonite, grey

limestone grit and loam show a back

scattering maximum (see also Fig. 2.10 with text). Note that the antisource point (the point just behind the source) is indicated in the figure by an arrow and by the break of data ( 8 factor

R

in the ordinate is a normalization factor.

of source is 53"). The

65 h

3 2 60C Q

CI

u

-

Woudgrond (air d r y )

small r i l l s medium r i l l s

-

a,

0 '

I

I

I

1000

I

2000 Wavelength (nm)

Fig. 3.9 Spectral reflectance of Woudgrond (with 7.4 %C and 12.0 X lutum) with fine and coarse rills after Damen (1975). The magnesium oxide surface which is used as a standard reflector is-assumed to

be a perfect Lambertian reflector. The reflectance of the standard surface, ( pst ) is independent of direction

and thus: Ist -1 I " The directional reflegtance p ( 8

I =

II

ISt and

p =

-=

,0)

is given by:

where p =

reflectance dependent on viewed nadir angle 0 of the reflectometer and position

0

of

the reflectometer with respect to the azimuth

0,

(principal plane) of the source; IO = intensity of the source; I and ISt are intensities of the radiation

=

180"

66

7

+

80

t

Red clay 60-

40-

D'

?

20-

Loam 80

40

0

40

@=OO Fig.

80 Nadir angle

(O)

4=180°

3.10 Directional

reflectance of various types of mineral surfaces 643 nm, 0 = 53" ) after Coulson (1966). (principal plane, h

Gypsum sand: Reach sand: Red clay: Limonite: Limestone: Loam:

translucent grains 0.1-0.5 mm translucent quartz grains 0.1-0.3 m 1 wn, aggregates 1-2 mm mean diameter 14 urn, range 3-40 um grey coloured rock crushed and graded to 1.2 cm size 1-5 pm, aggregates 50-1000 m.

reflected from the sample and from the standard surface respectively. Fig. 3.11 shows the directional reflectance of desert sand for radiation of different wavelengths and for different angles of incidence. There appears to be a strong increase of overall reflectance with increasing wavelength from 406 to 796 nm (fig. 3.11a),

which is in accord with the reddish COlOUK of

the

desert sand under consideration. The broad minimum reflectance near the nadir is a general characteristic of many surfaces. Apparently, the Lambert's cosine law (2-22),

which indicates a

hemispherical distribution is not valid for granular surfaces. Furthermore, the non-specular behaviour is evident at least in the forward direction, since the forward maximum is found at 0

> e0

( 0 =

e0

specular point).

The backscattering maximum will be due to surface reflection being composed of a diffuse component and a specular component. The latter component will be due

67 100

(a) ,

,

?

80

d

60

40

20

\'""

a, V

c fu

; 80

40

40

0

7

%

I

I

4J

80

80

I

I

I

0

40

80

0=l8Oo

O=O"

@=180"

I$=o"

I

40

CL

Nadir angle

I=

Fig. 3.11

("1

Nadir angle

Directional reflectance from desert sand after Coulson (1966): a) of five different wavelengths (principal plane, 0 = 53"); b) for three different angles of incidence (principa? plane, A nm)

.

to the presence of facets which are more

OK

=

("1

643

less oriented normal to the

direction of the incident radiation. The forward maximum for wavelengths between 492 nm and pronounced than the backscattering maximum (fig. 3.11a). for radiation with A presence

of

Fe

=

1025 nm is more

The opposite is true

405 nm. A possible explanation may be f o u n d in the

(III),

which

causes

absorption

and

strong

external

backscattering in this range. However, the same should apply to the radiation at 492 nm, which is not the case (for absorption, see Fig. 2.18).

Therefore,

the phenomenon is not completely understood. Both the total reflectance and the directional reflectance vary with the angle 0

at which the radiation is incident, as can be seen from fig. 3.11.b.

The variance is particularly pronounced at grazing angles.

In having the

highest reflectance at a grazing angle of 78,5", the surface acts in a non-

68

Lambertian way. The degree of

polarization of radiation reflected by desert sand in the

principal plane is shown in fig. 3.12. A maximum positive polarization (normal to

the principal plane)

was

found

120"

at

130"

to

from the antisource

direction, while vertical or negative polarization (parallel to the principal plane)

occurred

Furthermore,

a

in

the

region

considerable

surrounding

change

of

the

the

degree

antisource of

direction.

polarization with

wavelength is evident, the shorter wavelengths showing a higher degree (fig. 3.17.a).

40

(a)

(b)

30

20

10

0 -5

I

5

.r

+cc,

0

2

80

@=oO

I

1

40

0

40

80 @=180°

Nadir angle

Fig. 3.12

40

80

4.0"

("1

0

80

40 0=180°

Nadir angle ( " )

Degree of polarization of radiation reflected from desert sand after Coulson (1966) : a) for five different wavelengths (principal plane, 8 = 5 3 " ); b) for three different angles of incidente (principal plane, X = 492 nm) Degree of polarization = (Ih \/It, + $) X 100 ( % )

-

It appears that strongly reflected wavelengths are only weakly polarized, but high polarization is observed at wavelengths for which the reflectance is low

69 (compare

with

radiation

of

fig. a

3.11a).

Particles

particular

which

wavelength,

are

show

a

generally

high

reflectance

translucent

for

for that

r a d i a t i o n and c o n s e q u e n t l y have a h i g h c o n t r i b u t i o n of t h e i n t e r n a l component t o t h e t o t a l r e f l e c t a n c e . Thus any p r e f e r r e d o r i e n t a t i o n of t h e e l e c t r i c v e c t o r i s d e s t r o y e d by t h e i n t e r n a l ( m u l t i p l e ) r e f l e c t i o n .

On t h e c o n t r a r y , a b s o r b e n t

m a t e r i a l s have no o r only a s m a l l i n t e r n a l component and t h e r e f l e c t a n c e i s formed f o r t h e g r e a t e r p a r t by t h e c o n t r i b u t i o n of t h e e x t e r n a l component. Some f a c e t s a t t h e s u r f a c e w i l l produce t r u e s u r f a c e r e f l e c t i o n ,

which i s an

Consequently p o l a r i z a t i o n i s h i g h when a b s o r p t i o n

e f f i c i e n t polarizing agent. i s high ( s e e a l s o f i g . 2 . 7 ) .

In f i g . '3.12b, pattern

t h e e f f e c t of a n g l e of

shifts

with

source

i n c i d e n c e on p o l a r i z a t i o n i s shown. The

p o s i t i o n and

polarization i n the antisource-direction.

always

shows

vertical

or negative

Furthermore, t h e d e g r e e of p o l a r i z a -

t i o n i n c r e a s e s w i t h t h e a n g l e of i n c i d e n c e . Black loam s o i l ( w i t h a r e l a t i v e l y high c o n t e n t of o r g a n i c m a t t e r ) shows a low overall reflectance ( f i g . 3.13.a). in

Fig.

with

i n c r e a s i n g wavelength and a h a c k s c a t t e r i n g maximum

A c o m p l e t e l y o t h e r p a t t e r n a s conpared w i t h f i g .

3.13.h.

Although

b a c k s c a t t e r i n g maximum a t 8

the

total

=

53" and 8

reflectance =

78,5"

appears

'3.11h.

to

he

i s shown

lower,

i s f o r d e s e r t s a n d , w h i l e t h e forward maximum i s a l m o s t a b s e n t .

Fig.

shows

the

a

s t r o n g wavelength dependence and h i g h

wavelengths f o r b l a c k loam s o i l . 3.14.b.

The e f f e c t of

the

i s more pronounced t h a n i t

polarization changing 8

at

3.14.a shorter

i s shown i n f i g .

A s l i g h t s h i f t i n t h e n e u t r a l p o i n t s of p o l a r i z a t i o n toward t h e a n t i -

source-direction

may be noted f o r t h e b l a c k loam a s compared w i t h t h e d e s e r t

sand. It

will

be

evident

from

the

foregoing t e x t

that

there

o p p o r t u n i t i e s t o d i s c r i m i n a t e v a r i o u s s o i l s on t h e b a s i s of least

under

laboratory

conditions)

although

the

are excellent

reflectance (at

explanation

of

spectral

f e a t u r e s may be c o m p l i c a t e d .

---_------__

Thermal d a t a

The t e m p e r a t u r e of

a s o i l i s one of

i t s important p r o p e r t i e s i n c o n t r o l l i n g

Its

g e r m i n a t i o n of s e e d s , p l a n t growth and a number of s o i l forming p r o c e s s e s . importance

is

expressed

in

the

U.S.

Soil

Taxonomy

by

introducing

soil

t e m p e r a t u r e regimes i n t h e c l a s s i f i c a t i o n of s o i l s ( S o i l Survey S t a f f , 1975). The t r a n s f e r of h e a t i n t h e s o i l t a k e s p l a c e by c o n d u c t i o n through t h e s o l i d materials

and

across

the

pores

by

conduction,

convection

and

radiation

70

100

(b) 80

796ni

'I

60

't 40

L

m J 1

c,

-+ u al

80

1

1

1

40

0

40

I&$

ar

J

I

80 @=180°

80

I

I

I

I

40

0

40

80

Q=O"

@=180'

CL

Nadir angle

k

Fig. 3.13

('1

Nadir angle

('1

Directional reflectance of black loam soil after Coulson ( 1 9 6 6 ) : a) for four different wavelengths (principal plane, eo = 7 8 , 5 " ) ; b) for three different angles of incidence (principal plane, X = 6 4 3 nm)

together, as well as by latent heat transport (water vapour).

The effect of

soil mineralogical composition on the thermal behaviour may be evaluated from the emissivity values. In table 2.3, which,

in

the

8-14

carbonates, sulphates

a number of absorption bands are given,

pm region, are OK

due

to

the

presence

of

silicates,

phosphates. The effect of the composition of igneous

rocks on the emissivity

(E)

may be evaluated from Fig. 3.4.

composition may lead to difference in

E

of 0.15,

Difference in

while the emissivlty minimum

differs from the maximum by values of 0.15-0.3. E

values of soil materials for the spectra- region between 5 mn and 15 WI

are reported (by Idso e.a.,

1 9 6 9 ) to be from 0.95

for sand up to 0.97 for silty

clay and loam. Quartz sand shows a relatively low emissivity. Fuchs et al., ( 1 9 6 8 ) report fOt the socalled Plainfield sand the following values for three different moisture contents:

moisture content E

(emissivity)

0.7%

5.8%

8.4%

0.90

0.92

0.94

71

4[

I 0 '

3c

2c

10

0

-5

I

.l

I

I

I

I

I

I

I

0

40

1

80 @=180°

Q) 'r

Q)L

Nadir angle (")

s-m

Ez-,g

Nadir a n g l e (")

n a-

Fig. 3.14

Degree of polarization of radiation reflected from black loam soil after Coulson (1966) : a) for four different wavelengths (principal plane, 8 = 78,5"); b ) for three different angles of inciden2e (principal plane, b = 492 nm)

Monteith (1973) describes the thermal properties of soil as follows. If the volume fraction x of each component is expressed per unit volume of soil, one can write:

where s stands for solid, 1 for liquid and g for gaseous components of soil. The bulk density of a soil p ' is found by adding the weight of each component i.e.

p' =

where p x g g

pSxs + plxl + P x

g g

can be neglected.

( 3-3)

12

bulk specific heat C'.

It can be found by adding the specific heat (C) of the

soil components, as follows:

Van Wijk and de Vries (1963) present values on thermal properties of soils (which are also discussed by Monteith, 1973). They are given in table 3.1 (for definitions, the reader is referred to par. 2.7). The effect of soil composition on thermal properties can be evaluated to some Table 3.1

Thermal properties of soils and their components (after Van Wijk and De Vries, 1963) Density

Soil components Quartz Clay minerals Organic matter Water Air (20'C)

Specific Thermal heat conductivity

Therma 1 diffusivity

2.66 2.65 1.30 1 .oo 1.20~10-3

0.80 0.90 1.92 4.18 1.O 1

8.80 2.92 0.25 0.57 0.025

4.18 1.22

0.14 20.50

1.60 1.80 2.00 1.60 1.80 2.00 0.30 0.70 1.10

0.80 1.18 1.48 0.89 1.25 1.55 1.92 3.30 3.65

0.30 1.80 2.20 0.25 1.58 1.58 0.06 0.29 0.50

0.24 0.85 0.74 0. ia 0.53 0.51 0.10 0.13 0.12

1 .oo

Soils Water content g 0-3 Sandy soil (40% pore space) Clay soil (40% pore space) Peat soil (80% pore space)

0.0 0.2 0.4

0.0 0.2 0.4 0.0 0.4 0.8

extend from these data.

Quartz and clay minerals show similar density and

specific heat. Quartz sands tend to have larger thermal diffusivities than other soil types due to the high conductivity of quartz. Organic matter shows about half the density and more than twice the specific heat of quartz and clay.

Peat

soils

have

the

smallest

thermal

conductivity of organic matter is relatively small.

diffusivity,

because

the

73 Soil moisture is one of the most important factors influencing the thermal characteristics of soil. The thermal conductivity increases with increasing moisture content, but the increase becomes less marked with high moisture contents.

This

can be

components in table 3.1.

understood

if

one compares the data of the soil

The thermal conductivity of air is much lower than

that of water. Therefore, dry soils, in which air is filling the pores, have relatively low thermal conductivities. Raising moisture content will first result in much higher values due to the presence of thin waterfilms conducting the thermal energy. High moisture contents with a consequent almost complete filling of pores with water, only produces a slight increase in conductivity.

In dry soils, high porosity will lead to low thermal conductivity. The relation

! 40

!

20

30

I

I

50

60

! 70

80

Porosity

Fig. 3.15

(%I

Relation between soil porosity and thermal conductivity of dry soils after Chudnovski (1962).

Normally, the porosity of non-cultivated soils is at cultivated soils values of 40-60

30-40

%,

while in

% OK more are reached. Therefore, it may be

concluded that the increase of porosity by cultivation practices will lead to a decrease in thermal conductivity. The soil heat-flux shows seasonal and diurnal cyclic patterns. At mid latitudes, the greatest positive heat-flux the greatest negative heat-flux At

OCCUKS

OCCUKS

in spring and early summer;

in early winter.

low latitudes, the seasonal fluctuations are

relatively small.

Minor

74

fluctuations are illustrated in fig. 3.16 0,47"N

for Yangambi (Zaire) at a latitude of

and an altitude of 365 m above mean sea level. The curves show that the

60

2 50 W V

L a,

n

40

20

Fig. 3.16

Mean monthly soil temperature at a depth of 50 cm for the year 1952 at Yangambi, Zaire, and major climatic factors that affect it (Soil 1953). Survey Staff, 1975 after I.N.E.A.C.,

mean monthly soil temperature at a depth of 50 cm below the s o i l surface fluctuates with rainfall and amount of sunshine (dependent on cloud cover). Apart

form seasonal fluctuations, the diurnal fluctuations are of

importance for the process of heat exchange.

Fig.

3.17

great

shows temperature

profiles of s o i l and air near the ground for a clear midsummer day and night at mid-latitudes. Several features can be deduced in the diurnal temperature profiles of this figure, such as:

-

the

-

the diurnal amplitude of soil temperature oscillations is retarded with soil

the positive heat-flux with regard to the topsoil after sunrise; the strong positive heat-flux during the early afternoon; the negative heat-flux after sunset; there is an upward movement of heat; strong

negative

heat-flux

before

sunrise;

during

the

night

the

temperature of the subsoil is raised at the cost of the topsoil; depth.

15 Fig. 3.17 illustrates a specific case of radiation and soil profile; there are no general features as far as numerical values are concerned; the depths

indicated depend on thermal inertia.

h

E

140

U

v

U S

2 0 L 01

120 100

W

> 0 n

80

m

c,

r

01

60

.r

W

1

40 20

-

0

E V

v

-20

U S

2 0

L rn

x

0

-40 -60

7

a,

n r

c, P W

n

-80

-100 10

20

30

40 Temperature ("C)

Fig. 3.17

Temperature profiles of soil and air near the ground for a clear midsummer day and night at midlatitude (Fitzgerald, 1974 after Gates, 1970).

3.3.

Interaction of short wave radiation with plants Spectral reflectance .................... The reflectance characteristics and thermal emission of plants are

extensively discussed a.0. by Fitzgerald (1974). In this text only a summary is given. A cross section of a typical leaf is shown in fig. 3.18.

The upper and

76 lower

epidermis

have

a

mainly

protective

function with

regard

to

the

interaction with electromagnetic radiation, the mesophyll region i s the most important part. The mesophyll contains the plastid and vacuolar pigments. The plastid pigments are concentrated near the cell walls, while the vacuolar pigments are distributed uniformly throughout the cellular protoplasm. The most important plastids are the chloroplasts (disc-shaped, 5-8

Chloroplasts

Air cavity

Guard c e l l s

Air ca'vity Fig. 3.18

!J

m in

Upper epidermis

Lower epidermis

Scheme of morphological structure of a plant leaf after Fitzgerald (1974).

diameter and 1 chlorophyll diameter).

!J

is

m thick). Within the chloroplasts are grana, in which located

(these

grana

are

0.5

!J

m

long and

0.054

!J

m

in

Chlorophylls form about 65 X of the leaf pigments. The other most

common plastlds are the carotenoids, which are yellow to orange red in colour, and are the chief colourants in plant leaves in the absence of chlorophyll. The main vacuolar pigments are anthocyanines and anthoxanthlns, which are

normally red and b l u e In colour. The colour of leaves in autumn I s due to

71 carotenoids,

whether

OK

not

in

combination

with

anthocyanines

and

anthoxanthins. The upper portion of the mesophyll, the palisade layer, consists of closely packed elongated cells with their long axis perpendicular to the leaf surface.

It shows a concentration of chloroplasts, which strongly absorb the Visible radiation. The

lower mesophyll, the

arrangement and

spongy mesophyll, has a

less compact cellular

the intercellular spaces are larger as compared with the

palisade layer. This is the main transpiring tissue of the leaf, which shows a low concentration of chloroplasts and has a light green colour. Fig. 3.19 shows a typical reflectance spectrum of a normal healthy plant leaf together with the absorption spectrum of water.

Wavelength (pm) Fig.

3.19.Reflectance spectrum of a typical green leaf and the absorption spectrum of water in the spectral range of wavelengths 0.4-2.6 LI m after LARS (1968).

78

The spectrum of a plant leaf can be divided into the following ranges: the range between 0.4 !J m and 0.7 p m, which is characterized by

-

very low reflectance due to intense absorption of the incident

-

radiation by pigments in the plant; the range between 0.7 u m and 1.3 u m, which is characterized by

-

very little absorption and high reflectance; the range between 1.3 u m and 2.6 u m, which is characterized by pronounced minima.

The reflectance and absorptance of plant leaves is mainly determined by: the pigmentation; absorptions are caused by electron transitions

-

within

the pigment molecular

pigments absorb at 0.43 band at about 0.66

!J

-

0.44

complexes (Fitzgerald,

1974);

all

m, but chlorophyl has an additional

IJ

m; reflection of green radiation is produced by

the chloroplasts; the mesophyl structure, which causes multiple reflection of near Infrared radiation at cell walls; the water

content; the most

intense absorption bands occur at

1.4 IJ m and 1.9 IJ m;

the surface properties of the leaf; a matt surface approximates a perfect Lambertian diffuser more closely than a glossy surface does; the latter shows a prominent specular component in the reflected radiation. Normally, the energy absorbed is converted by photosynthesis into heat and stored energy. absorbed

energy

However, chlorophyll is capable of by

fluorescence.

There

are

two

releasing some of types

of

the

chlorophyll:

chlorophyll-a and -b. Chlorophyll-a shows strong absorption maxima at 0.43 IJ m and 0.66 IJ m. Chlorophyll-b shows a strong absorption maximum at 0.64

IJ m.

In

most plants the content of chlorophyll a : b is 3 : 1, so chlorophyll-a largely determines the absorption. Both types of chlorophyll show typical fluorescence spectra, namely: chlorophyll (a) with maxima at 6 6 8 nm and 723 nm; chlorophyll (b) with maxima at 648.5 nm, 672 nm and 7 0 5 nm. The spectral reflectance of leaves undergoes strong changes both early and late in the growing season. Gates ( 1 9 7 0 ) provides data on these changes for Quercus Alba (white oak). The data are presented in Fig. 3.20.

79

-2

100

aJ

I

I

V

E

m

c, V

60

40

20

I

0.4

r

h

J

I

0.5

0.6 I

1

I

0.7 I

1 0.8 I

! 0.9 I

I

1 .a

I 1.1 I

I

L

- 19 -- 20 - _ - _ _18 .......... 18

40

20

7

t

01 0.4

21 28

2

-.-

June July August September October October October November

I

I

0.5

0.6

,

I

0.7

I

I

I

0.8

0.9

I

1 .o

1.1

Wavelength (pm) Fig.

3.20 Changes in spectral reflectance throughout the growing season of leaves of Quercus Alba after Gates ( 1 9 7 0 ) .

The juvenile leaf has a dense covering of pubescence and shows a relatively high reflectance in yellow and red, and a very high reflectance in near Infrared. As the leaf grows and expands, the hairs spread out, the near

80

Infrared reflectance drops, some absorption takes place by chlorophyll and the green reflectance increases.

Later on a further increase in blue and red

absorption accompanies a slight reduction in green reflectance and a visible darkening of the leaf (May 11 and 18). surfaces

has

At this time, the number of reflecting

increased, resulting in an

increase of

the near

Infrared

reflectance. During the growing season from May 18 to October 21, curves remain nearly constant. On October 28,

the spectral reflectance

the end of the growing season

presents itself by a break-down of chlorophyll, as is illustrated by the shift of the green peak towards yellow and orange wavelengths (Anthocyanins are formed which absorb blue and green).

Furthermore, there is a reduction in

reflectance at 0.8 u m upon pronounced drying. However, the reflectance at 1.0

u m remains constant. The curves of fig. 3.20 give a good impression of

these changes in leaf reflectance throughout the growing season.

In remote sensing, however, we have to deal with the reflectance of a plant canopy,

which

is

determined

by

plant

variables

such

as

leaf

area

and

orientation of leaves and stems. It is obvious that there are often differences in spectral response between plant canopies. Fig. 3.21 illustrates this in showing spectra of grass, birch, pine and fir canopies. Murtha (1978)

describes the damaging agents which may be active in a forest,

and the manifestation of the damage itself. The following damaging agents are mentioned: insects, disease, fire, water deficits, flooding, air pollution, storms, activities of recreationists and beavers. The manifestation of damage may be: a change in morphology (e.g.

growth reduction, defoliation, loss of

branches, cellular collapse or wilted look);

-

a change in physiology (e.g.

-

OK

decrease in photosynthates, deterioration of

chloroplasts, interruption of translocates including water); both.

It can be concluded that the effects of damage on spectral reflectance will be one

OK

both of the following:

-

in case of a change in morphology, a decrease in overall reflectance of

-

in case of a change in physiology (chronic damage over a l o n g period),

the plant especially in the near Infrared; a

shift of the green peak towards yellow wavelengths due to a deteroriation of chloroplasts and finally a shift towards red wavelengths.

81

Grass

Birch

100

Fig.

3.21

Coulson

600

800 Wavelength (nm)

Reflectance spectra of four types of plant canopies after Krinov (1953; also given by H o l z , 1973).

(1966) has studied the directional reflectance of grass; a brief

discussion is given below. The directional reflectance of green grass is shown in fig. 3.22.a.

One of the

features is the low reflectance of radiation with wavelengths of 492 nm and 643 nm, which is in accord with the data presented above (fig. 3.19-3.21).

The

broad minimum reflectance of 796 nm and 1025 nm radiation near the nadir and the asymmetric shape due to forward scattering are further characteristics. Note that the asymmetry is much less than for a number of soil materials e.g. gypsum (fig. 3.11). Fig. 3.22.b

shows the directional reflectance of green grass at different

angles of incidence of light at a wavelength of 643 nm. At Bo backscattering maximum

=

78.5",

a

is observed, which is in accord with the absorbent

nature of plant leaves for radiation of this particular wavelength. Curves of the polarization of radiation reflected from green grass are given in fig. 3.23. The radiation that is strongly reflected by the cell walls of the

82

100

(b)

(a) 80 -

60 -

,? i

40 -

$,,=78.5

O

c

t

I

,

f 0'

20 -

40

80

0

40

@=OO Nadir angle (")

k

Fig. 3.22

80 @=180°

Nadir angle

("1

Directional reflectance of green grass (grass stands thick, height of grass 4-5 cm) after Coulson (1966): a) at four different wavelengths (principal plane, 0, = 53"); b) at three different angles of incidence (principal plane, A = 643 nm). Note: for explanation see par. 3.2.

palisade tissue (A

=

796 nm and A = 1025 nm)

but the radiation reflected by

shows little polarization,

the chlorophyll (A

=

492 nm and A

=

643 nm)

shows a high degree of polarization. Radiation with a horizontal polarization is not absorbed so strongly by the chlorophyll as radiation with a vertical polarization. Thus owing to preferential absorption, the reflected radiation is polarized. Fig. 3.23.b

shows the degree of polarization at different angles of incidence.

The pattern shifts with the position of the source and shows a negative or a

vertical polarization in the antisource direction. The degree of polarization increases with the angle of incidence. An anomalously high polarization appears at O o = 78.5"

. The maxima are located closer to the antisource direction than

in the curves of soil materials presented in fig. 3.12 and Fig. 3.14.

83

40

(b)

20

I 0

-5

'*. \

0'

-

1025nm

.

I

I

I

1

I

80

40

0

40

80 @=180°

@=O" N a d i r angle ( " )

Fig. 3.23

N a d i r angle

("1

Degree of polarization of radiation reflected from green grass after Coulson (1966) : a) at four different wavelengths (principal plane, 9 = 53"); b) at three different angles of incidence (principaf plane, X * 492 nm) Note: for explanation see par. 3.5.

Looking at objects from the source direction means a simplification i n canopy variables, since shadows are not visible. This direction is called the "hot spot". Bunnik (1978) points to the value of the "hot spot" for measurement of the canopy reflectance. Thermal properties _____-__-_____--__ Beyond 2 u m, the reflectance of plant leaves is very low. The leaves behave at longer wavelengths of the near Infrared almost as black bodies, the emissivity being about 0.97.

To prevent the plant from reaching high

84 temperatures, the leaves radiate efficiently at wavelengths longer than 2 u m. Healthy

plants

are

in energy

equilibrium with

their environment.

Their

temperature is adjusted when environmental parameters change s o that the loss of energy is equal to the gain of energy. In the formulae 2-31

and 2 - 3 2 ,

the

energy budget equations are given. Some environmental parameters that influence the energy budget are: the relative humidity of the air, the air temperature and the wind speed. The characteristics of the plant that are important in this connection are the width of leaf (or other characteristic dimensions) and the diffusion resistance (in s m-').

These parameters may be used to formulate the

exchange of energy by convection between the leaves and the air, and the transpiration rate of water from the leaves (see Gates, 1970).

The plant

moisture condition has a pronounced influence on the leaf temperature at a specified intensity of solar radiation. The moisture condition of the plants may be expressed in the relative turgidity (RT), which is defined by (Namken, 1 9 6 4 ) :

RT

= 100

(FW - DW/TW

-

DW)

(3-5)

where FW is the field condition weight of leaf samples, DW is the weight of the leaf samples after drying at 60°C and TW is the turgid weight achieved by floating the leaf samples on distilled water overnight under illumination. In fig. 3 . 2 4 ,

the air temperature at the time of the measurements ( 2 : 3 0 -

3.:00 pm) on the two dates differed by 3.5

K and the relative turgidity of the

cotton leaves equilibrated with a change in radiation intensity in about 45 sec. The data show that

the thermal response of the leaves to changing

radiation is linear; the standard errors of estimate (Sy.x)

indicate that leaf

temperatures could be estimated within 0.9 K, two thirds of the time. The variable plant moisture conditions in Fig. 3.25

were achieved by

timing of irrigation during a rainless period. At the mid-afternoon measuring time, the cotton-plant leaves exhibited wilting symptoms at about 70 percent RT; at 6 0 percent RT, the leaves were extremely flaccid. The data in Fig. 3.25 indicate that cotton-leaf temperature under the specified conditions can vary about 3.5 K from RT

=

6 0 X to RT = 8 2 X around the wilting point.

The difference between bare soils and plants is pronounced in the diurnal changes in temperature. In general, plants are cooler than soil during day time, and warmer during night time. A strong difference between plant and soil will occur around noon on a clear day.

85

I

V

O" 7

4(

k W

6/1/65 = 31.8k.4 C = 67.9*2.5% = 9.08 C(ly/min)-' = .898

TA

L

c 3 ,

RT

m L

g 3E

b

W

r2

+J

I

0 0

Ic 5 W J

36

34

32

b

6/1,2,3/64 = 28.3k1.3 = 77.7k1.7 = 9.9

r2

=

.go6

sy+

=

.91

TA RT

30

/ 0

28

Fig.

3.24

I

I

I

I

0.6

0.8

1 .o

1.2

I

1.4 1.6 S o l a r radiation, Rs(ly/min)

Influence of solar radiation on cotton-leaf temperature (Namken, 1 9 6 5 ) . (Permission Am. S O C . o f Agronomy, I n c . )

Some remarks are made in conclusion. Gates ( 1 9 6 4 ) reports about the difference between conifers and broad-leafed deciduous plants. Conifers are cooler than broad leaves during day time and warmer during night time under similar conditions. The reason is that the fine needle structure of conifers increases the

convection

efficiency

and

couples

the

conifers

tightly

to

the

air

temperature. At night, deciduous leaves will c o o l down to several degrees below the air temperature by radiation. Pronounced thermal contrasts occur between deciduous and evergreen trees in the autumn season. Seasonal variations in plant temperature have been found to depend largely

86

v

t.

41

1

I

I

A

aJ L

3rn

I

40

L

aJ

TA

= 33.9k.7

RS

=

1.32k.04 -.15

a

b

=

* aJ %

r2

=

E

.864

39

aJ -I

38

37

36 58

Fig. on

3.25

I

I

1

I

I

I

62

66

70

74

78

82

86

The effect of relative turgidity on leaf temperature (Namken, 1965)(Permission Am. SOC. of Agronomy, Inc.)

the seasonal variations in moisture availability. For a discussion on the variation in plant temperature with external

factors, the reader is referred to Fitzgerald (1974). temperature of

leaves and plant canopies and

Both

the absolute

the temperature differences

between leaves and the ambient air are of interest (Gates, 1970).

The former is

of interest for the rate of biochemical reaction and the moisture condition of plants, the latter may be used in comparing effects of treatment.

3.4.

Implications for remote sensing The reflectance data of minerals (par.

3.1.)

are essentially obtained

under laboratory conditions. In rocks and soils we are normally concerned with assemblages of minerals and consequently the discrimination potential with regard to mineralogy is lower. Therefore, only rough estimates may be obtained, but

this

can

be

sufficient

for

the

detection

of

concentrated

mineral

occurrences. The t e x t s on s o i l s and v e g e t a t i o n pay a t t e n t i o n t o p r o p e r t i e s such a s o r g a n i c m a t t e r c o n t e n t , a n o r g a n i c c o m p o s i t i o n , m o i s t u r e c o n t e n t , roughness of s o i l s and c o m p o s i t i o n a s w e l l a s s t r u c t u r e of

plant

leaves.

However,

remote s e n s i n g

p r o v i d e s d a t a on s o i l s and r o c k s a s a whole, and p l a n t s a s c o v e r t y p e s , r a t h e r than the individual constituents.

From remote d i s t a n c e s one c a n n o t e x p e c t t o

obtain

individual

detailed

information

on

constituents,

although

laser-

t e c h n i q u e s o p e r a t i n g w i t h c o h e r e n t h i g h i n t e n s i t y EMR may form a n e x c e p t i o n t o t h i s s t a t e m e n t . The rough remote s e n s i n g d a t a , .however, have t h e a d v a n t a g e t h a t they o f f e r a v e r a g e f i g u r e s f o r a n assemblage of a s p e c t s o v e r a r e l a t i v e l y l a r g e surface area.

These f i g u r e s a r e d i f f i c u l t t o o b t a i n on t h e ground s i n c e t h e y

r e q u i r e a tremendous amount of o b s e r v a t i o n s and samples. An example of a rough

is

estimate

the

so-called

albedo,

which

represents

the

total

radiant

- r e f l e c t a n c e of n a t u r a l o b j e c t s . R a r r e t t and C u r t i s p r e s e n t s e v e r a l v a l u e s ; some of t h e s e a r e g i v e n i n t a b l e 3.2.

T a b l e 3.2

Albedo v a l u e s of v a r i o u s n a t u r a l s u r f a c e s ( R a r r e t t and C u r t i s , 1976).

~~t y p e of s u r f a c e

albedo, r e f l e c t e d r a d i a t i o n a s X of i n c i d e n t r a d i a t i o n

soils

37 14 14 8 86-95 20-29 16-23 18 17 14 12-13 10-14 10

snow vegetation

f i n e sand dry black s o i l moist ploughed f i e l d moist black s o i l dense c l e a n snow d e s e r t shrub land w i n t e r wheat oaks deciduous f o r e s t pine f o r e s t prairie swamp v e g e t a t i o n heather

Conclusions

3.5.

The

spectral

vibrational origin.

reflectance

features

An example of

The l a t t e r i s r e p r e s e n t e d by a band a t 1.4 and 2.5 Soils

u may

m f o r C03". show t h e s e

are

either

of

electronic

IJ

m f o r OH',

characteristics,

but

of

u

in

m and 1.9 addition

m.

u

bands hetween 1.6

and t h e w a t e r a b s o r p t i o n hands a t 1.4 reflectance

or

t h e former i s a broad i r o n hand a t 1.1

m

m. present

v a r i a b i l i t y due t o s u r f a c e roughness a s i n f l u e n c e d by s o i l t e x t u r e , s t r u c t u r e

88

and tillage. Furthermore, the organic matter content strongly influences the spectral reflectance e.g.

a high organic matter content results in an overall

low reflectance. Plants show a typical reflectance spectrum as influenced by canopy structure, pigmentation, mesophyll Structure, water content and surface properties of the leaf. A l l pigments absorb at 0.44

!J

absorption band at 0.66

Normally, green is reflected more strongly

IJ

m (red).

m (blue),

but chlorophyll also shows an

than blue and red. The palisade tissue of the mesophyll with its large area of cell walls is mainly responsible for the high reflectance of near Infrared by plant leaves. Furthermore, the surface properties of the leaf have great influence on the reflectance as is illustrated a.0. by white oak leaves in their juvenile stage (pubescence). The work of Coulson (1966)

with regard to the directional reflectance of

natural surfaces reveals interesting features, such as the following: highly absorbent materials (e.g.

plant leaves, and soils rich in organic matter)

deviate from low absorbent materials (which show a forward peak in scattering) in having a backscattering maximum. With

regard

to

emission,

emissivity minima between 9.0

!J

the

following can

m (acid rocks) and 10.5

be !J

stated:

rocks

show

m (ultrabasic rocks).

For rocks as well as for soils, the diurnal temperature change is the most significant short-term variation that is usable in remote sensing. The transfer of heat in the soil takes place by conduction, convection and radiation together, and by latent heat transport (water vapour).

Soil moisture

is one of the most important factors influencing the thermal hehaviour of soil. The difference between soils and plants is most pronounced in the diurnal changes in temperature. In general, plants are cooler than soil during day time and warmer during night time. Seasonal variations in plant temperature largely depend on the seasonal variation in moisture availability. 3.6.

References

Barrett, E.C. and Curtis, L.F., 1976. Introduction to Environmental Remote Sensing. London, Chapman and H a l l : 336 pp. Bowers, S.A. and Hanks, R.J., 1965. Reflection of Radiant Energy from Soils. Soil Science, Vol. 100, No 2. The Williams & Wilkins Co, U.S.A.: pp. 130-1 38.

Bunnik, N.J.J., 1978. The multispectral Reflectance of Shortwave Radiation by Agricultural Crops in relation with their Morphological and Optical Properties. Thesis Agric. Univ., Wageningen, The Netherlands: 176 pp.

89

Chudnovski, A.F., 1962. Heat Transfer in the Soil. Israel Program for Scient. Transl. (Transl. from Russian), Oldbourne Press, London. 1971. Cipra, J.E., Baumgardner, M.F., Stoner, E.R. and MacDonald, R.B., Measuring Radiance Characteristics of Soil with a Field Spectroradiometer. Soil Sci. SOC. Amer. Proc., vol 35: pp. 10141017.

Condit, H.R., 1972. Application of Characteristic Vector Analysis to the Spectral Energy Distribution of Daylight and the Spectral Reflectance of American Soils. Applied Optics, Vol. 11, No 1: pp. 74-86.

Coulson, K.L., 1966. Effects of Reflection Properties of Natural Surfaces in Aerial Reconnaissance. Applied Optics, Vol. 5, No 6: pp. 905-917. Damen, J.P.N., 1975. Poging tot verklaring van Reflectiespectra van een serie Bodemmonsters, gemeten met de Niwars-spectrometer. Niwars-publ. NK. 25: 56 pp. Fitzgerald, E., 1974. Multispectral Scanning Systems and their Potential Application to Earth-Resources Surveys. Spectral Properties of Materials. ESRO CR-232, Neuilly, France: 231 pp. Fuchs, M. and Tanner, C.B., 1968. Surface Temperature Measurements of Bare Soils. Journal of Appl. Meteor., Vol. 7. Gates, D.M., 1964. Characteristics of Soil and Vegetated Surfaces to Reflected and Emitted Radiation. Proc. of the 3rd Int. Symp. on Remote Sensing of Environment, Univ. of Michigan, Ann Arbor: pp. 573-600. Gates, D.M., 1970. Physical and Physiological Properties of Plants. Chapter 5 , Remote Sensing, pp. 224-252; produced by the Committee on Remote Sensing for Agricultural Purposes. Publ. Nat. Acad. of Scces. Girard, M.C. and Girard, C.M., 1977. T616d6tection de la Surface du S o l . ler Colloque P6dologie T616d6tection, Rome: pp. 55-64. Gold, A. and Asher, J.B., 1976. Soil Reflectance Measurement using a Photographic Method. Soil Sci SOC of Amer. Journal, Vol. 40, No 3: pp. 337-34 1.

Higham, A.D., Wilkinson, B. and Kahn, D., 1973. Multispectral Scanning Systems and their Potential Application to Earth-Resources Surveys. Basic Physics & Technology ESRO (European Space Research Organisation): 186 pp.

R.K(ed), 1973. The Surveillant Science. Remote Sensing of the Environment. Houghton Mifflin Cy, Boston: 391 pp. Hunt, G.R., Salisbury, J.W. e.a., 1970-1976. Visible and Near Infrared Spectra of Minerals and Rocks I/XII. Modern Geology, Gordon and Breach, Science Publ. Ltd. Belfast, N-Ireland. Idso, S.B. and Jackson, R., 1969. Comparison of Two Methods for Determining Infrared Emittances of Bare Soils. Journal of Appl. Meteor., Vol. 8. Institut National pour l'Etude Agronomique du Congo Belge et du Ruanda-Urundi, Ann6e 1952. Bur. Climatol. Commun. 7: 144 pp. Janse, A.R.P., Bunnik, N.J.J., 1974. Reflectiespectra van enige Nederlandse Bodemmonsters bepaald met de Niwars-veldspectrometer. Niwars publ. No 18: 3 1 pp. Janse, A.R.P., Bunnik, N.J.J. en Damen, J.P., 1976. Reflectiespectra van enige Nederlandse Bodemoppervlakken. Landbk. Tijdschr. Jg 88, NK. 8: pp. Holz,

254-260.

Krinov, E.L., 1953. Spectral Reflectance Properties of Natural Formations. Acad. of Scces, USSR. Nat Res. Council of Canada. Techn. Transl., TT-439. Lars, Laboratory for Agricultural Remote Sensing, 1968. Remote Multispectral Sensing in Agriculture. Purdue Univ, Agric. Exp. Stat., Res. Bull., Vol. No 3: 175 pp.

90

Lyon, R.J.P., 1965. Analysis of Rocks by Spectral Infrared Emission (8-25 u m) Economic Geology, Vol. 60: pp. 715-736. Mathews, H.L., Cunningham, R.L. and Petersen, G.W., 1973. Spectral Reflectance of Selected Pennsylvania Soils. Soil Sci SOC. her. Proc., Vol. 37: pp. 421-424. Meyer-Arendt, J.R., 1972. Introduction to Classical and Modern Optics. Prentice-Hall Inc., Englewood Cliffs, N.J.: 558 pp. 1973. Principles of Environmental Physics. Contemporary Monteith, J.L., Biology. Edward Arnold (publ.) Ltd London: 241 pp. 19781. Remote Sensing and Vegetation Damage: A Theory for Murtha, P.A., Detection and Assesment. Symp. on Remote Sensing for Vegetation Damage Assessment 1978. Publ. by Amer. SOC. of Photogrammetry: 32 PP. Namken, L.N., 1964. 'Ihe influence of crop environment on the internal water balance of cotton. Soil Sci. SOC. Amer. Proc. 28: pp. 12-15. Namken, L.N., 1965. Relative turgidity technique for scheduling cotton irrigation. Agron. J. 47: pp. 38-41. Obukhov, A.I. and Orlov, D.S., 1964. Spectral Reflectivity of the Major Soil Groups and Possibility of using Diffuse Reflection in Soil Investigations. Soviet Soil Sci. 1964: pp. 174-184. Orlov, D.S., 1966. Quantitative Patterns of Light Reflection by Soils. I. Influence of Particle (aggregate) Size on Reflectivity. Soviet Soil Sci. 1966: pp. 1495-1498. Planet, W.G., 1969-1970. Some Comments on Reflectance Measurements of Wet Soils. Remote Sensing of Environment 1, Elsevier N-Holland: pp. 127129.

Skidmore, E.L., Dickerson, J.D. and Schimmelpfennig, H., 1975. Evaluating Surface-Soil Water Content by measuring Reflectance. Soil Sci. SOC. Amer. Proc., Vol 39: pp. 238-242. Soil Survey Staff, 1975. Soil Taxonomy. A Basic System of Soil Classification for making and interpreting Soil Surveys. U . S . Dept of Agric. Handbook No 436: 754 pp. Stoner, E.R., Baumgardner, M.F., Biehl, L.L. and Robinson, B.F., 1980. Atlas of Soil Reflectance Properties. Agric. Exp. Stat. Purdue Univ., West Lafayette, Indiana, Res. Bull. 962: 74 pp. Wijk, W.R. van, Vries, D.A. de, 1963. Periodic Temperature Variations. In: Physics of Plant Environment, ed. van Wijk, North Holland Publ. Co., Amsterdam. 3.7.

Additional Reading

Bunnik, N.J.J. and Verhoef, W., 1974. The Spectral Directional Reflectance of Agricultural Crops. Niwars Publ. No 23. Colwell, J.E., 1974. Vegetation Canopy Reflectance. her. Elsevier Publ. Cy. Remote Sensing of Environment 3: pp. 175-183. Gausman, H.W. and Cardenas, R., 1968. Effect of Pubescence on Reflectance of Light. Proc. of 5th Symp. on Remote Sensing of Environment. Univ. of Michigan, Ann Arbor: pp. 291-297. Hoffer, R.M., Johannsen, C.J., 1969. Ecological Potentials in Spectral Signature Analysis. In: Remote Sensing in Ecology. Unlv. of Georgia Press, Athens: pp. 1-16. Karmanov, I.I., Rozhkov, V.A., 1972. Experimental Determination of Quantitative Relationships between the Color Characteristics of Soils and Soil Constituents. Soviet Soil Sci. 1972: pp. 666-674. Knipling, E.B., 1970. Physical and Physiological Basis for the Reflectance of Visible and Near Infrared Radiation from Vegetation. her. Elsevier

91 Publ. Cy, Remote Sensing of Environment 1: pp. 155-159. Koolen, A.J., 1979. Temperatuurbeelden van Onbegroeide Grond op weg naar Landbouwpraktijk? Landbk. Tijdschr. pt. 91, Nr 9: pp. 258-264. Leu, D.J., 1977. Visible and Near Infrared Reflectance of Beach Sands: A Study on the Spectral Reflectance/Grain Size Relationship. Remote Sensing of Environment 6 , Elsevier N-Holland: pp. 169-182. Scharringa, M., 1976. Temperatuurklimaat van de Bodem. Landbk. Tijdschr. pt. 88, Nr. 8: pp. 261-264. Suits, G.H., 1972. The Calculation of the Directional Reflectance of a Vegetative Canopy. her. Elsevier Publ. Cy, Remote Sensing of Environment 2: pp. 117-125. Suits, G.H., 1972. The Cause of Azimuthal Variations in Directional Reflectance of Vegetative Canopies. Amer. Elsevier Publ. Cy, Remote Sensing of Environment 2: pp. 175-182. Torres, 1973. La Thermographie Questions Techniques et Problemes de 1'InterprGtation. Revue Photo-InterprGtation 1973-2: pp. 48-73. Verhoef, W. and Bunnik, N.J.J., 1974. Spectral Reflectance Measurements on Agricultural Field Crops during the growing season. Niwars publ. No 31: 7 2 pp. 1975. A Model Study on the Relations between Verhoef, W. and Bunnik, N.J.J., Crop Characteristics and Canopy Spectral Reflectance. Niwars publ. No 33: 89 pp. Verhoef, W. and Bunnik, N.J.J., 1976. The Spectral Directional Reflectance of Row Crops. Niwars publ. No 35: 134 pp. Vincent, R.K., Rowan, L.C., Gillespie, R.E. and Knapp, C., 1975. Thermal Infrared Spectra and Chemical Analysis of Twenty-six Igneous Rock Samples. Remote Sensing of Environment. Amer. Elsevier Publ. Cy: pp. 199- 209.

Watson, R.D., 1972. Spectral Reflectance and Photometric Properties of Selected Rocks. Remote Sensing of Environment 2: pp. 95-100.

This Page Intentionally Left Blank

93

4.

DETECTION OF ELECTROMAGNETIC RADIATION

Life

earth is dependent

on

on

solar radiation and has developed systems

that use solar radiation as an energy source (e.g.

plants) and as a means of

visual perception (man and a great number of animals).

Two of our five senses

are capable to detect EMR these being the eye and the nerve endings. The latter sense heat. The eye enables vision and is of primary interest to our purpose, since

it

is

difficult

to

think about methods

for remote inventory and

monitoring of the natural environment that do not depend some phase

of

on

the human eye in

the processing or interpretation. The only alternative is

braille! Some methods of image interpretation require a certain ability of human vision. One

of these, the ability to get a stereoscopic impression of overlapping

images is normally present. Another requirement connected with the study of coloured images is correct colour vision. The different aspects of human vision are discussed in par. 4.1.

To expand o u r view, that is to make visible, radiation to which the eye is not

sensitive, may

be

done by

photographic

as well

as non-photographic

techniques with detection capability in the zones of the EM spectrum outside the Visible (see par. 4.2 and 4 . 3 ) . After this first subdivision in photographic and non-photographic methods of detection, attention is given in par.

4.4.

to the different types of

platforms on which detectors may be mounted. Finally in par. 4.5, a discussion is presented on ground-investigations. The latter have to be directed to the remote

sensing

tool

and

therefore

deviate

partly

from

conventional

investigations. 4.1.Human vision The eye is capable of sensing radiation of wavelengths between 0.4 and 0.7

m (or more precisely 380-760 nm),

the so-called Visible zone of the EMS.

As is stated above, human vision has to be used in one or more steps of processing or interpretation, so that some understanding of it is necessary. There are a number of aspects connected with vision that have to be dealt with in this context, namely:

-

colour perception; stereopsis or depth perception,

94

-

resolving power. The construction of

the eye is to some extent comparable with

the

photographic camera, since one as well as the other possess a diaphragm, a lens and a sensitive layer. The light enters the eye through the cornea, which is separated from the l e n s by fluid; the maximum light refraction occurs at the cornea. The iris is the pigmented part of the eye that controls the aperture (pupil), which can be varied over a ratio 16:l. The lens is active in accomodating or focusing for near and far vision. For this, the shape of the l e n s can be modified by varying the tension on the membrane attached to its margin. For nearby vision, the tension is released and the lens gets a more convex shape as compared with its shape for far vision. The image is focused on the retina, which contains the light receptors, the so called rods and cones. The rods and cones differ in threshold as is indicated in Fig. 4.1

and serve under low illumination (e.g.

twilight) and under high

illumination (e.g. daylight) respectively.

000

\ \

I

400

Fig. 4.1

500

600

700 Wavelength (nm)

Threshold responses of retinal receptors (after Land, 1977)

95

The spectral sensitivities of rods and cones are presented in Fig. 4.2. The curve that peaks at about 500 rn corresponds to the sensitivity of rod pigment. The other three curves correspond to the cone pigments and show peaks respectively at 435 nm, 520-550 nm and 550-595 nm. The latter extends into the long wavelengths up to about 6 5 0 nm, thus enclosing orange as a whole. Note that the sensitivity ranges of the cones are overlapping each other. The maximum concentration of receptor cells is found in the fovea. Close to the fovea is the so-called blind spot. At this place, the optic nerve joins the eye and there are no receptor cells. h

100-

N W .r 7

m

E

80-

0 E

v

4> 2,

60

-

.r v)

2

I-’

40-

.r E

%

.r

20-

c,

m

7

a W

0 400

Fig.

500

600 700 Wave1 ength (nm)

4.2 Normalized spectral sensitivities of four visual pigments (after Land, 1 9 7 7 ; adapted from work of Brown and Wald of Harvard Univ.)

The so-called retinex theory (Land, 1 9 7 7 ) helps to explain COlOUK vision. Retinex is used for the ensemble of biological mechanisms that convert flux into a pattern of lightnesses. The experiments of Land show that objects are observed in the same colour even under a great variation of illumination intensity. Therefore, flux does not appear to be the defining factor. In human vision, the COlOUK sensation is made less dependent

OK

even independent on

flux, since a comparison is made in the retinex system of lightness of a specific area with respect to the lightnesses of its surroundings. Although the activation of two retinex systems has been found to be sufficient for COlOUK sensation, normally three retinex systems will be active. A t

96 daylight, the three cone pigments act and determine individually the lightness of an object or area. The colour of the object or area is a result of the report on the three specific lightnesses and as stated above of the comparison of these with the lightnesses of its surroundings. The visual pathways (see Kaufman, 1 9 7 4 ) , that is the pathways from the eyes to the central nervous systems, are given in fig.

4.3.

The fibers

comprising the optic nerve may be thought of as divided into two intermixed bundles. One bundle of the optic nerve contains fibers originating from cells at

the temporal side of

the eye, and the other bundle contains fibers

originating at the nasal side of the eye. The fibers that originate from the temporal sides go to the hemisphere of the brains at the same side of the head as the eye in which the fibers originate. The nasal fibers cross over, that is they go to the opposite hemisphere of the brains.

left

right

object

RH

brains

/ \ / I

LH Fig. 4 . 3

Pathways from the optic nerves to the central nervous system.

L E , RE = resp. left and right eye LH, RH = resp. left and right cerebral hemisphere

The eye produces, images that are upside down. The left side of an object will be at the right side of the image on the retina, and the right side of an object at the left side of that image. In other words: the left side of an object is projected nasal for the left eye and temporal for the right eye. Consequently, points to the left of the scene produce signals in the right cerebral hemisphere and those of the right side of the scene in the left cerebral hemisphere (see fig. 4 . 3 ) . The crossing of signals to contralateral hemispheres plays an important part in binocular depth perception, since i t enables fusion of double images in the binocular field of view. It has been known for many years, that fusion is not really a necessary

97

condition for stereopsis (Kaufman, 1974); called

pictorial

cues

involve

light

and

there are several cues. The soshade,

texture, interposition,

perspective and relative size. Others are:

-

kinetic cues, the motion parallax

OK

the difference in imaging between far-

away objects and nearby objects when the head is moved, and the kinetic depth effect that is the systematic transformation of retinal images by movement with respect to the object;

- physiological cues, being the accomodation of the lens and the convergence and divergence of the eye axes. For stereoscopic observation of airphotos using parallax differences of image objects, however, fusion is a must. The eye shows normal aberrations (Davson, 1962), namely:

-

spherical aberrations, that is the rays from an object-point entering the eye at different points of the cornea are not bent to converge at a unique point

on the retina;

-

chromatic aberrations, that is the focus for blue rays is before the retina, and for red rays behind it, while yellow rays are in focus.

Furthermore, there are the individual aberrations and deficiencies with respect to perception of

CO~OUK,

texture and pattern (see Julesz, 1975 and Young,

1964). The resolving power of the eye is determined by the largest diameter of its receptor cells (Sabins, 1978).

The maximum diameter, which mounts 3 u m,

has to be multiplied by the refractive index of the vitreous humor (n obtain the effective diameter as expressed by

a'

(=

= 1.3)

to

the angle or radian

measure of the outer rays in the eye that compose the retinal image).

The image

distance from the retina to the lens is about 20 nun. The effective width of the receptors therefore is approximately 4f20.000 or 1/5.000 of the image distance. Since

a'

is proportional to a ( = the angle between the outer rays coming from

the margins of the scene into the eye (Davson ed.,

1962), the effective width

can be placed upon 1f5.000 of the object distance as well. Therefore, adjacent objects must be separated by l/5.000 of the object distance in order to fall on alternate receptors. However, the detection capability of the eye is influenced not only by the size of the objects but also by their shape, contrast and orientation (Sabins, 1978). The fibers going from the retina to the brains have to carry information

98

about position, shape, size, texture, brightness and colour. This is only possible by unique codes. According to Kaufman (1977), the natures of

the codes are not conclusive

OK

the theories concerning even fully convincing.

Therefore, we will only discuss one of these sensations in a rather simple hut practical way, namely: colour vision. The colour sensations blue, green and red are called the primary colours. Combination of green and red light produces yellow; blue and red light produce magenta, while blue and green light produce cyan. Yellow, magenta and cyan are the so-called secondary colours. Considering the properties of secondary colour dyes: the observation of yellow (ye),

magenta (ma) and blue-green or cyan (cy) means specific absorption of

respectively blue (bl), green (gr) and red (re).; of respectively green

+

red, blue

+

red and blue

the dyes show transmittance

+

green. The properties are

expressed in the colour circle and can be used for the composition of colours (fig. 4 . 4 ) .

a

b

C

Fig. 4 . 4 The Colour circle: a) basic division (after Gerritsen, 1972); b) mixing of primary colours; c) mixing of secondary colours (b and c after Smith, 1968); For abbreviations: see text. (Used by permission of Am. SOC. for Photogrammetry and Remote Sensing.) White is produced by mixture of the three primary colours. The super-position of three secondary colour dyes produces grey to black, since blue, green and red are absorbed. Colour is a composite three-dimensional characteristic consisting of a lightness attribute and two chromatic attributes, being hue and saturation (Hunter, 1975).

Hue is the colour sensation associated with different parts of

the spectrum denoted by blue, green, red, cyan, yellow (,orange) and magenta.

99 Saturation or chroma is the colour sensation which corresponds to the degree of hue in a colour. The arrangement of colours in a hue and saturation surface is given in fig. 4.5a.

Full saturation and complete absorption by the secondary COlOUKS (as

meant in fig. 4.4~) is found on the outer circle.

WH!TE

BLACK

(a)

(b)

Fig. 4.5 Arrangement of colours (Hunter, 1975) a) Hue and saturation surface; b) Three-dimensional COlOUK space. (Reprinted by permission of John Wiley & Sons, Inc.) Lightness or value is equivalent to some member of the series of achromatic colour perceptions ranging for light diffusing objects from black to white, and for regularly transmitting objects from black to perfectly clear and colourless (Wyszecki e.a. fig. 4.5b.

1967).

A three-dimensional colour space is given in

The lightness dimension provides an achromatic center axis, on

which the hue circle can be positioned at varying lightness levels. Using the absorption characteristics of the secondary COlOUK dyes, that is

subtraction of

light of specific wavelength range, is known as the

subtractive way of colour formation (fig. 4.4.~).

this in contrast to the

additive way being the addition of light of specific wavelength (fig. 4.4.b). Actually, the eye can only operate on an additive way.

Suppose blue is

subtracted from white light by passing through a filter. Thus green and red light are transmitted, which produce the same effect as the addition of green and red light would do, that is they add to yellow. Often it is found difficult to understand colour formation both in a subtractive and an additive way. However, both ways are essential to the production of colours by photography. The

effect of different quantities of the secondary colours yellow,

magenta and cyan dotted over a white reflective surface is illustrated in the

100 ITC Colour Chart (plate 3 ) . The pigments dotted on the white paper surface subtract light from the incident radiation. Yellow pigments subtract blue light, magenta pigments subtract green light etc. The transmitted light is reflected by the paper surface. Representing this light by vectors, as is done in fig. 4.6,

enables a schematic

presentation of subsequent additive colour formation. For more information on Colour Science, one is referred to text books such as Judd and Nyszecki ( 1 9 6 3 ) and Wyszecki and Stiles ( 1 9 6 7 ) . 4.2.

Photographic techniques Photographic techniques may be used for detection of a portion of the

Ultraviolet (0.3-0.4

p

m), of Visible radiation (0.4-0.7

of the near Infrared range (0.7-0.9

I . !

m).

!J

m) and of a portion

Filters and specific films are used

to obtain information in broad wavelength zones or in relatively narrow bands. There are three categories of filters (Slater, 1 9 7 5 ) :

-

antivignetting filters;

-

polarization filters.

spectral filters; Antivignetting filters are usually produced by depositing a metal alloy

on glass in such a way that the central area of the filter is absorbent and the

circumferential

region

is

transmittent.

They

are

used

to

improve

uniformity of image-plane irradiance (Slater, 1 9 7 5 ) . Spectral filters are divided into absorption and interference filters. Absorption filters can be produced by cementing gelatin between planeparallel plates of optical quality glass. The gelatin has an admixture of organic dyes. Resides gelatin filters, filters of example, absorption curves of presented in fig. 4.7. 200-400

coloured glass are available.

the Kodak Wratten

filters 1A and

As an 2A are

The 2A filter is a complete absorber for radiation of

nm, while 1A shows 1 X transmittance over the 310-380 nm wavelength

zone. Furthermore, the large variety of Kodak Wratten filters is demonstrated in fig. 4.8. Interference filters comprise quarterwave optical-path- thickness layers of alternating high and low refractive-index materials. Unwanted radiation is to

be

reflected and

canceled, while

the

required

radiation is

to

be

transmitted. Polarization

filters consist

of

a

coating of

polarization film on

(optical quality) glass. The position of the filter can be such that only the

101

B1

Gr

I

I

Re

0

I

B1

\

\

\

\

\

\

\

\

Gr

\

i

/

I

; B?

/

I

I

I

/

I

uGr/

/

/

YeGr’

/

\

\

u d 1

=

/

/

/

\ Abbreviations :

/ \

/

\

/

unsaturated

= dark =

light

ye = yellow Ma = magenta Cy = cyan B1 = blue Gr = green Re = red e.g.

ReMa = reddish magenta.

Fig. 4 . 6

. I

Re/

/

’

\

I

cr

Additive Colour formation schematically.

I

102 component parallel to the principal plane is transmitted. The principal plane is the vertical plane containing the sun, the ground target and the observer straight ahead (plane of incidence; see also fig. 2.5).

Polarization filters

may be used to enhance image quality in the presence of haze. Haze produces multiple reflection at randomly oriented particulates. The

.I

100

1A

200

300

400

500

600

700 800 900 Wavelength (nm)

.I

2A

W U S

3 1 c, .r

E u)

5 L

10

k-

i100

300

400

500

600

700

800

900

Wavelength (nm) Fig. 4.7 Spectrophotometric transmittance curves of Wratten filters 1A and 2A (Eastman Kodak Cy, 1970).(Reprinted courtesy of Eastman Kodak Company.) scattered light shows polarization dominantly horizontal, that is perpendicular to the plane of incidence. By transmitting only the component parallel to the plane of incidence the effect of haze will be reduced and the image contrast will be enhanced. Filters require a correction on exposure time in order to compensate for the radiation removed by the filter. The factor, by which the exposure with filter has to be greater than the exposure without filter, is called the filter factor. The main photographic film-types are:

-

panchromatic or black and white films sensitive for the wavelength range

103 from 360 to 720 nm;

-

Infrared black and white films recording Visible and near Infrared radiation (up to 900 nm);

Fig. 4.8 Spectral-transmittance bar charts for Selected Kodak Wratten Filters courtesy of Eastman Kodak Company.) (Eastman Kodak Cy, 1970).(Reprinted

104

-

true colour films sensitive for blue, green and red light;

-

false colour films recording green, red and near Infrared radiation (up to

900 nm). All the films mentioned when applied from airborne or spaceborne platforms require filters to

remove unwanted

radiation and

to

improve image-plane

irradiance. In table 4.1 some film-filter combinations are given. To produce Infrared black-and-white photography, a large part of the Visible is often excluded from reaching the film. This may be done by application of light-red

OK

dark-red

filters. A yellow filter is applied to obtain false colour photography. Table 4.1 Examples of film-filter combinations Wratten filter no

Excluding wavelength (nm) below”

Film

Eliminating effect of

1A (sky light)

310-386

2A (pale yellow)

406-413

true colour film true colour film

G12 (yellow OK minus blue)

492-508

weak haze, flying altitude up to about 3000 m moderate to strong haze, flying altitude more than 3000 m strong haze

15G (dark yellow)

508-520

25A (light red) 89B (dark red)

580-590 680-703

*

panchromatic film and false colour film Infrared black strong haze and white film item item item item

first figure 0.1 %, second figure 10 % transmittance curves Eastman Kodak Cy (1970).

transmittance according to

Photographic films consist of a flexible transparent base (film base) coated with one or more emulsion layers of which each has a thickness of approximately 100

J !

m. The emulsion is a suspension of silver halide grains in

solidified gelatin. The

panchromatic

emulsion layer, while

and

Infrared

black-and-white-films

consist

of

the true colour and false colour-films have

one three

emulsion layers. The grains of the emulsions are a few micrometers in diameter (Sabins, 1978) and have an irregular shape. They have been processed to increase their sensitivity to light. On exposure, when a proton strikes one of the grains, an electron is trapped at an imperfection of a grain, it may

105 convert a silver ion into a neutral silver atom. If more than one photon is received by a silver halide grain within about a second, combinations of four atoms of silver are formed, which are stable. Thus by interaction of light with silver halide grains a latent silver image is formed. By the reduction process in developing panchromatic films, the exposed grains are converted into opaque grains and unexposed grains are removed leaving clear areas in the emulsion. The parts of the scene with low reflection will show up bright, the parts with high reflection dark and moderate reflecting areas grey; it is a negative image. When such a film is printed onto photographic paper, the signatures are reversed and a positive image is the result. Recently a new film type has been

introduced in which during the

developing process, silver is replaced by a black dye. The resultant product is less granular when compared with the silver image and enables therefore high enlargement factors. However, no application in aerial photography of this new type of film is known t o the author. The characteristics of the emulsion layers in true COlOUK and false colour-films are similar to those of panchromatic film with the following additions:

-

each layer has a maximum sensitivity, this being for true colour film in the blue, green and red bands, and for false colour film in the green, red and near Infrared bands,

-

during developing each emulsion layer forms a COlOUK dye, that is for true colour-film complementary to

the blue, green and red radiation that

exposed the layers being yellow, magenta and cyan, for false colour-film yellow in the green sensitive layer, magenta in the red sensitive layer and cyan in the near Infrared sensitive layer. The false COlOUK-fih is described in table 4.2 by comparing it with the true colour-film. In true colour-film, a yellow filter between the blue and green sensitive layers prevents blue from reaching the underlying layers, which are also sensitive to blue radiation. Blue is eliminated from the false colour-film by the use of a yellow filter on the camera. The blue sensitive (yellow forming) layer of the true colour film is replaced by a near Infrared sensitive layer in the false colour film, and the dyes to be produced are shifted one position, tnat is yellow for the green sensitive layer, magenta for the red sensitive layer and cyan for the near Infrared sensitive layer. So do the

106 Table 4.2 Comparison between true colour-film and false colour-film. layers

1st layer

--------2nd layer 3rd layer

true colour-film

false colour-film (with yellow filter on camera)

sensitive fOK

sensitive for

dye

resulting colour

near Infrared

cyan

red

green red

yellow magenta

blue green

dYe

blue yellow yellow filter green magenta red cyan

resulting colours in the false colour photograph, respectively blue for green, green for red and red for near Infrared reflecting objects. By developing a true colour film, a yellow image is formed at the exposed places in the blue sensitive layer, the green sensitive layer forms a magenta image etc. These images are negative images, because the original radiation intensity is transformed in dye. For instance, much blue results in much yellow dye, which transmits little blue. When the negative film i s projected onto photographic paper, a positive colour print is produced. In this example: the negative shows much yellow, which transmits little blue, producing little yellow on the positive, which transmits much blue (that is the original intensity).

In fig.

4.9,

a schematic presentation is given of the colour

negative process and the production of colour positives of true colour, as well as false colour-film. Grass is taken as an example; for the reflectance of grass, the reader is referred to fig. 3.21.

The incident radiation and the

radiation transmitted by the film positives are indicated by vectors. Since blue is not included in the false colour-film and replaced by near Infrared, which is not visible to the eye, false colours are produced. The amount of dye formed in the positive products is proportional to the incident radiation in such a way that much radiation produces little dye. Aerial colour films are often of the reversal type, since these films produce a high contrast. Therefore, some attention is given below to the processing of these films (Slater, 1975). The first step in processing of colour reversal film is the development in a black and white developer. A negative image is formed in each layer. Fogging exposes the remaining silver halide. This is normally accomplished

true colour f i l m

1

1

1

radiation

qrass

illum. with white 1i g h t

i11u m i n a t i on of f i l m

NIR 4 r.

bl

gr

re

dark green c o l o u r strong absorption

bl + re moderate a b s o r p t i o n g r

p o s i t i ve

false colour f i l m

7

BB

-

1 minus

bl

1

grass

NIR

1 gr

4

-

re

re

bl

magenta

gr

-

red colour

v e r y much r e

re

moderate a b s o r p t i o n b l strong absorption g r positive

Fig.4.9

bl

Schematic presentation of colour formation in true and false colour films using grass as an example

+

r e combine t o ma

108 The f o l l o w i n g s t e p is t h e development of

chemically i n r e v e r s a l processing.

t h e f i l m i n a c o l o u r d e v e l o p e r r e s u l t i n g i n t h e p r o d u c t i o n of dye i n t h e areas where t h e remaining s i l v e r h a l i d e i s reduced. A t t h i s s t a g e , a p o s i t i v e c o l o u r image and a n e g a t i v e s i l v e r image a r e produced. The f i l m i s t h e n bleached t o remove t h e s i l v e r images b u t l e a v i n g t h e dyes u n a f f e c t e d . A g r e e n s u b j e c t w i l l show up b l u e when viewed i n t r a n s m i t t e d l i g h t because magenta and cyan dyes

are formed, which a b s o r b g r e e n and r e d l i g h t . The s p e c t r a l s e n s i t i v i t y c u r v e s of two t y p e s of Aerochrome I n f r a r e d f i l m 4.10.

are g i v e n i n f i g .

It can be n o t e d t h a t t h e cyan-forming-layer

f i l m s is slower a s compared w i t h obtain

a

lower

sensitive layer s e n s i t i v i t y of Infrared

response

to

the other layers.

near

Infrared

i n the

This h a s been done t o

radiation.

The

near

Infrared

is t h e upper l a y e r i n t h e f a l s e c o l o u r f i l m and shows a 2 a b o u t 0 ( = 1 e r g / c m ) i n a r e l a t i v e l y broad zone of n e a r

radiation,

being

very

low

when

compared

to

the

green

and

red

s e n s i t i v e l a y e r s ( y e l l o w and magenta forming l a y e r s ) .

3

x

2.0

>

.r

2

1.0

ul

.r 4J

yellow forming layer

.r

w

2.0

.-

flj\

magenta forming layer

yellow forming layer

.r

cyan forming layer

c

W ul

a,

u l o

\

4J W

c

magenta forming layer

-1.0

#&

cyan forming layer

0

cn

,” -2.0

I

I

0.4

0.6

I

I

I

0

S -4.0

0.8 1.0 1.2 Wavelength (pm)

0.4

0.6

0.8 1.0 1.2 Wave1 ength (pm)

4.10 S p e c t r a l s e n s i t i v i t y c u r v e s f o r Aerochrome I n f r a r e d f i l m 2443 ( a ) and f o r High D e f i n i t i o n Aerochrome I n f r a r e d f i l m SO-127 ( b ) a f t e r S l a t e r ( 1 9 7 5 ) . The s e n s i t i v i t y is t h e r e c i p r o c a l of t h e energy i n ergs/cm2 ( 1 e r g = 100 n J) of monochromatic r a d i a t i o n t o produce i n t h e i n d i v i d u a l l a y e r an e q u i v a l e n t n e u t r a l d e n s i t y of 1.0 when t h e f i l m i s g i v e n n e g a t i v e processing. (Used by p e r m i s s i o n of Am. SOC. f o r Photogrammetry and Remote S e n s i n g . )

Fig.

The n e a r I n f a r e d l a y e r i s a l s o s e n s i t i v e t o V i s i b l e r a d i a t i o n .

However,

t h e q u a n t i t y of V i s i b l e r a d i a t i o n t h a t can be c a p t u r e d by t h i s f i r s t l a y e r i s ignorable

when

compared w i t h

the quantities

that

can be

absorbed

by

the

f o l l o w i n g l a y e r s of t h e f i l m , which show h i g h s e n s i t i v i t i e s t o g r e e n and r e d radiation.

S i n c e much of

the

green

i s c a p t u r e d by

the

second l a y e r ,

the

109 overlapping in sensitivity of the red sensitive (third) layer with the green sensitive (second) layer is of minor influence. ordering

of

the

layers

in

the

false

Therefore, owing to the

colour-film,

a

rather

accurate

registration of red radiation (in the magenta forming layer) can take place. The reduced effect of near Infrared radiation in the false colour-film results in a

better registration of natural objects, of which many are strong

reflectors of near Infrared, especially vegetation. At normal sensitivity, red would predominate in most cases over the whole photographic scene (objects with high near Infrared reflectance produce no or little cyan, therefore the film transmits much red). Because of strong scattering of Ultraviolet (UV)

by the atmosphere,

little application is found in remote sensing of W photography. However, a typical film-filter combination for UV photography may be mentioned, being the Kodak Plus-X Aerographic film 2402 with the Kodak Wratten 18A filter. The latter transmits the energy of the spectral range between 0.3

and 0.4

l~ m.

Special quartz lenses have to be used in order to transmit UV radiation also below the critical wavelength ( 0 . 3 5 4.3.

II

m) for glass of most cameralenses.

Non-photographic techniques Other remote sensing techniques than aerial photography are required for

detection in the wavelength range of the Infrared larger than 0.9

p

m. This

range covers solar radiation in the near and middle Infrared and earth emission in the middle and far Infrared (see fig. 2 . 2 ) .

Alternative ways of

detection are needed, because lens systems cannot be used for focusing the relatively broad spectral regions which are required for remote sensing in these low energy regions. An alternative way for detection of long wavelength radiation (as well as

for short wavelength radiation) was found in the so-called airborne line scanner. The

multispectral airborne

line-scanner

(fig.

4.11)

collects energy

in

distinct wavelength ranges (channels or bands) of a scene below in a series of scanlines each of which is perpendicular to the line of flight. The energy is received by a rotating mirror. The rotation of the mirror is adjusted to the velocity of the airplane in order to prohibit overlap or gaps between adjacent scanlines.

110 The mirror reflects the energy into a collector; the energy is divided into distinct bands and focused on a series of detectors. The scene is normally built up of objects that vary in reflection or emission properties and can be reconstructed when variations in signal strength get an address on the scan-

b ) Scanner schematic

a ) Scanning procedure during flight

Fig.

4.11 The

multispectral airborne optical Lillesand and Kiefer (1979). (Reprinted by permission of John Wiley

mechanical &

scanner

after

Sons, Inc.)

line. The signals can be stored on tape or displayed on a TV screen (or cathode ray tube)

as dark or

light tones. When a photographic film is

transported at the same speed as the repetitive lines pictured on the TV screen, a one-band photographic image can be derived as well (Rudd, 1974). The technique is called multispectral scanning,

OK

MSS.

Detectors can be classified according to Baker et al., (1975) into:

-

thermal detectors, based on increase of the temperature of heat-sensitive materials; the signal is a result of the absorption of radiation, which produces

a

variation

electrically;

in

the

detector

material

that

is

monitored

111

-

quantum-type detectors, based on the direct interaction of the incident photon with the electronic energy levels within the detector material.

The quantum-type detectors can further be classified into (Baker et al., 1975):

-

photoemissive detectors, in which the absorption of photons from incident radiation exites electrons within the sensitive material in such a way that they are emitted through a Surface barrier; these detectors operate in the Visible and Near Infrared up to about 1 u m wavelength;

-

photoconductive and photodiode detectors; incident photons with an energy greater than the energy gap of the detector material produce free-charge carriers, which cause the resistance of the photosensitive material to vary

in an inversely proportional ratio to the number of

incident

photons; these detectors are sensitive to wavelengths up to several 10's of micrometres; they may be composed of lead salts (PbS and PbSe).

Next to the airborne line-scanner some other non-photographic techniques have been developed. In image cameras

OK

TV-tubes like the Image Orthicon, the

scene is focused by a lens on a photoemissive cathode. This TV tube was developed in the late 1 9 3 0 ' s . The so-called Vidicon tube, based

on

photoconductivity, is the most

widely used camera tube today. It is relatively small, low in cost and has a long life-time. There are also modifications of Vidicon, such as: the Return Beam Vidicon (RBV),

Plumbicon and Silicon diode array camera tube. For more

information, one is referred to Baker et al. Recently, a new concept has been developed, by which sensing, storage and transfer can be done in a simple structure: the charge-coupled device (CCD) and more specific the charge-coupled imager (CCI).

A CCD consists of a linear

array of closely spaced MOS (= metal-oxide semi-conductor) capacitors formed by depositing metal electrodes over an oxidized silicon substrate. The CCD operates by storing information in the form of carrier charge packets, in the capacitors at the Si-Si02 interface. These charge packets, which are generated by absorption of the incident photon flux, are transferred serially to the output element by a multiphase clock. A serial output is produced representing the variation of the incident flux across a line (Baker et al.,

1975).

In

addition the CCI has an imaging device and seems to offer good prospects for use as a multispectral strip camera. Both the Vidicon and CCI are imaging sensors. A non-imaging sensor is the

112 radiometer, which measures the intensity of EMR emanating from objects within its field of view and sensitivity range. Radiometers operate in the Infrared spectrum at

wavelengths larger than

1 IJ m

(photometers operate at shorter wavelengths).

and

in the Microwave

region

The radiometer for measurement

of the Infrared requires a stable internal reference, since errors may result from the often variable radiation derived from components of the radiometer itself. The output of the detector in a radiometer is an electrical signal that

is

related to

the radiance difference between the target and

the

reference radiation. A so-called spectrometer is a radiometer which has a dispersing element that enables measurement as a function of wavelength. A n example is the NIWARSspectrometer. This spectrometer, designed for research and constructed by the Institute of Applied Physics (TNO, Delft, The Netherlands),

is based on the

simultaneous measurement of the radiant intensity of a standard reflector and of the object (Bunnik, 1978).

The spectral range of this spectrometer is

between 3 6 1 nm and 2360 nm. The bandwidth for the spectral ranges 361-753 nm, 629-1226

nm and 1165-2360 nm is resp. 17 nm, 25 nm and 42 nm. The detectors

are respectively Si for the first two intervals, and PbS for the last interval. A reflectance spectrum is determined by the object-reference ratio and by means of wavelength calibration of the three spectral intervals for all the grating positions. The final data are stored on magnetic tape and a hard copy of each spectrum is presented by a table and plot print. 4.4.

Remote sensing from various platforms Remote sensing can be done from different platforms, these being:

a)

ground-borne platforms, observation stations like towers and other high

b)

airborne platforms, being balloons, aircraft and rockets;

c)

space-borne platforms including satellites and other spacecraft.

buildings;

Ad. a) The groundborne platforms are generally used in specific studies that intend later application in airborne or space-borne missions. Ad. b) Free-floating balloons may be used that have an attractive stability. The balloon's

altitude can be controlled by using ballast drops and gas

valving, while a trajectory control can be fullfilled to some degree by knowledge of wind pattern.

113 Tethered balloons may be used for particular operations e.g. in archeology or in forestry. Different payloads can be applied that may be controlled by radio from the ground.Blimps, or observation balloons, are dirigable lighter-thanair craft, mainly used by the news media as aerial television camera platforms. Aircraft present a common type of remote sensing tool. Some convential aircraft used for remote sensing are: Cessna 337, Lockheed YO-3A

(see Colvocoresses et al.,

1975).

Beechcraft Bonanza A36 and But also unconvential types

may be used such as helicopters, drones (unmanned aircraft) and sail planes.

In the atmosphere, the aircraft are subject to vibration created by the engines or other parts of the aircraft, and distortions due to both the dynamics of the aircraft and the atmosphere. For this, corrections may be necessary in preprocessing the remote sensing data. Upwards the atmosphere becomes more stable, a high stability being reached at an altitude higher than about 150 km. Ad.

c)

The

history

(Colvocoresses et al.,

of

rockets

1975).

dates

back

as

far

The development of V-2

as

the year

1891

rockets in the Second

World War gave rise to a renewed interest, which resulted in the development of spacecraft. The launch of Sputnik in the year 1957 marked the definite start of remote sensing from space, although already in 1946 space pictures were taken by a photographic camera mounted on a V-2 rocket. Several NASA-missions into space have been performed:

-

unmanned spacecraft Nimbus program 1958 up to now, TIROS satellites 1960-1965, ERTS satellites (Landsat) 1972 up to now, ATS satellite 1974,

SMS satellite 1974;

-

manned spacecraft e.g. Skylab 1973.

Other programs of NASA e.g.

the Apollo flights were mainly directed to the

observation of other planets and the earth's moon. Remote sensing of the earth's

surface was

incidental in

these

programs,

yet

often

of

great

importance to the development of remote sensing. Specifications of a number of satellites are presented in table 4.3. specifications comprise the

The

name, country, operational period, altitude,

inclination, repetition period, wavelength bands, spatial resolution and the hour of daytime coverage.

114

m

m 3

m 3

01

m

T1

u 0

..I U

.I I

Y -.

E

a Y .4

0

I

Y .4

I

Y .A

1

E 3

Y

x

D m

'0

N

4

m m

B N

x

n .9 N

I 0 N

n 01

d

m

0

4 3

h

m

.

OI YI

d m

J

h

very high high moderate low very low

3.0

2.0 0.5

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