245
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Multitemporal analysis of Thematic Mapper data
for soil survey in Southern Tunisia
G.F.Epema
Department of soil science and geology, Laboratory for remote sensing, Agricultural University of Wageningen, Netherlands
ABSTRACT: The possibilities of Thematic Mapper data for the examination of the dynamics of an area in Southern
Tunisia are the subject of this study. A general view is given over the methods to compare Thematic Mapper
data of different days. It was shown that different methods and products are complementary for interpretation.
The most important changes were in salt content, vegetation and exposition. Recommendations and a start for
further research in order to find more subtle changes and for a more quantitative approach havd been
described.
1. INTRODUCTION
The dynamics of an area may give problems in
relating satellite or other remote sensing data to
surface characteristics. At the other hand the
dynamics of the surface are an important
characteristic of the units in an area. Also in
arid regions with a very sparse vegetation cover
important permanent and non permanent changes of
the surface may occur.
In this paper the possibilities and constraints
of multitemporal analysis of Thematic Mapper data
for the study of the dynamics of an arid area in
Southern Tunisia (near Kebili) are presented. An
extensive description of the area and of the use of
Thematic Mapper for mapping this area have been
presented by Mulders and Epema (1986) and Epema
(1986). Therefore only a very general description
will be given. The study area has an arid climate
and covers both the margin of a playa and
footslopes. Cuesta ridges are present with in front
of it the footslopes, which are partly covered with
gypsum crust. Moreover dunes of different heights
are present on the footslope and in the playa
border zone. In this last zone also spring mounds
are present. The playa and playa border zone are
saline flats with a groundwater depth less than 1
meter. With the exception of the oases the cover
with vegetation is small and in the playa even
absent.
For this area Thematic Mapper tapes were
available for 29 January 1983 and 18 May 1985.
These days represent the situation after
respectively relatively dry and wet circumstances.
Changes in multitemporal remote sensing data can be
made visible with an interpretation of images of
the different days. In order to make a valuable
interpretation the type of image and limits of
linear stretch must be appropriate. Therefore the
spectral characteristics of the units have to be
studied first. Then an overview is given of
relevant possible differences between these two
dates. These are due both to changes of the surface
itself and to the effect of difference in sun
elevation and turbidity of the atmosphere at
Landsat overpass time. Different methods are
compared in order to match the data in such a way
that relevant changes in spectral reflectance can
be read from the data. Then spatial differences in
dynamics of reflectance will be shown.
Thematic Mapper for 29 January 1983 and for 18 May
1985. The digital numbers on these tapes were
converted to reflectance values at the top of the
atmosphere. With the aid of tables presented by the
NASA (Technical Memorandum 86149; 1984) and more
recent reports, characteristics of Thematic Mapper
of Landsat 4 and 5 on these two days for the
channels (or bands) could be gathered. A procedure
leads to linear transformation from digital numbers
to reflectance values with only a small intercept.
Feature space plots, correlations between
spectral bands and principal components were
studied in order to get insight in the spectral
behaviour of the surface. The feature space plots
in fig. 1 are useful to show the main spectral
groups.
2. SPECTRAL CHARACTERISTICS
Computer compatible tapes were available of
Fig. 1. Plots of May reflectance at the top of the
atmosphere for band 4 TM with bands 1, 3 and 7.
