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Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
Assessment of soil properties from spectral data 
G.Venkatachalam & V.K.R.Jeyasingh 
Indian Institute of Technology, Bombay 
ABSTRACT: Application of computer aided analysis of Landsat data to the finer levels of identifying soil character 
istics, such as grain size, is beset with difficulties because of the poor resolution of the Landsat data. The difficulty 
is essentially due the low correlation between grain sizes of soils and their reflectance values which precludes predic 
tion of grain sizes from reflectance to a satisfactory degree. Hence, an attempt has been made in this study to evolve 
mathematical models which give better correlations between the gradation of soils and their spectral reflectance chara 
cteristics. 
The first model is in the form of a linear relationship between the grain sizes and the third Principal Components 
of the reflectance data. A second model with a higher predictive ability has also been suggested based on the concept 
of Primary, Secondary and Tertiary Principal Components. Further, using non-linear optimization techniques, a third 
model having a very high correlation has also been evolved. These models were based on a group of soil samples, colle 
cted from different parts of India. 
The model obtained through optimization was found to have the best predictive ability. Therefore, the optimization 
technique has been extended to a larger sample group of fourteen different soils. The accuracy of the model so evolved 
has been tested statistically and the model applied successfully to predict grain sizes of a few soils collected from 
certain other parts of the country. 
This study will help in the application of Landsat data for estimating grain size characteristics of soils, if their 
responses in the four bandpasses are known. 
1. INTRODUCTION 
Spectral reflectance characteristics of soils vary broadly 
due to many factors such as colour, mineral content, 
covertype, moisture content, texture or grain size, 
type of parent rock and land-forms. Multispectral sca 
nner (MSS) data collected from space-borne platforms 
have been successfully used in exploration for dema 
rcation of land-forms, soils and rock types on a broad 
basis. But, studies related to soil characteristics such 
as grain size are limited because of the poor resolution 
of the MSS data. 
A good amount of research work has been reported 
in estimating soil texture in terms of sand, silt and 
clay fractions using microwave remote sensing. Lab 
oratory studies by Gerberman (1979) showed the exis 
tence of a definite linear relationship between sand 
levels and their reflectances in the visible and near 
infra-red regions, in a mixture of sand and clay. MSS 
data in the visible and near infra-red regions could 
not be effectvely used to study the variations in soil 
texture and other characteristics because of their poor 
correlations. Since a number of orbiting platforms 
are routinely collecting information in the visible and 
near infra-red regions, it is worthwhile to evolve a 
method to use this information to study the soil charac 
teristics. Hence the present investigation has been 
taken up with an objective of evolving a relation- ship 
between the grain size characteristics of naturally 
available surface soils and their reflectance values 
measured in the laboratory in the visible and near 
infra-red regions of the electromagnetic spectrum (EMR) 
by siutable transformation techniques. 
their relfectance values corresponding to the four bands 
of the Landsat data. 
3. Analysis of the data to evolve a reliable model 
by suitable transformation techniques to predict the 
grain size characteristics of soils. 
4. Testing the model to find the accuracy of predic 
tion. 
2.1 Experimental setup 
A Cema sungun with an operating temperature of 3 50 0 
degrees Kelvin was used as a light source. 
An Exotech model 100-A Landsat ground truth radio 
meter was used as a sensor. It observes the reflectance 
in four bands corresponding to Landsat except for band- 
7, where the range is 0.8 to 1.0 micron. 
A barium sulphate coated plate was used as a standard 
reference surface. It was assumed to have 100 percent 
reflectance and specular in nature. 
2.2 Experimental investigations 
Initially, five types of soils from two different states 
in India, with ten samples for each type, have been 
used for reflectance measurements in the laboratory 
under oven dry conditons. The average reflectance 
values of these samples are given in Table 1. For these 
five soils the grain size characteristics were also deter 
mined. These data have been used for arriving at the 
three models stated above. In the second phase, similar 
experiments have been carried out on a larger group 
of 14 soils representative of a variety of Indian soils. 
2 APPROACH AND METHODOLOGY 
The present investigation has been carried out in the 
following four stages: 
1. Formulation of an experimental setup in the lab 
oratory where observations could be taken under contr 
olled conditions. 
2. Collection of soil samples and measurement of 
2.3 Statistical analysis 
The primary objective of the statistical analysis was 
to transform the original reflectance data with a view 
to improve the correlation between the grain sizes 
and their transformed reflectance values. The well 
known Principal Component transformation has been 
used. The four band data have been transformed