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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Multi-temporal Landsat for land unit mapping on project scale
of the Sudd-floodplain, Southern Sudan
Jonglei Executive Organ, Karthoem, Sudan
H.A.M.J.van Gils
International Institute for aerospace survey and earth sciences, Enschede, Netherlands
ABSTRACT: The Sudd floodplain is extremely flat, covered mainly by grasslands, only locally interrupted by
clusters of fields and seasonlly flooded by rain and/or river water. Since photo-interpretation is based on
relief, vegetation structure and field pattern, such technique is not satisfying for the Sudd floodplain.
Moreover, the most important environmental component - the flooding - can be assessed only on sequential series
of photographs.
Studies of the Sudd-floodplain on regional scale have been carried out as environmental impact study for the
Yonglei Canal project. However, such information proved of limited value on project scale. For the latter a
combination of Landsat, aerial photography and field survey has been tried and proofed successful. The methodo
logy, results and limitations are outlined for application in similar floodplain areas as there are the Llanos
in Southern America, Zambesi floodplain, Kafue flats and the Okavango swamps in Africa.
The available remote sensing material for the area
was at the time of the survey (1983):
- Black and white panchromatic aerial photography
scale 1:40 000 from december 1980.
- Computer Computable Tapes (CCT) of four sequential
Landsat MSS Scene 186/055
Of these the first two and the last are used for
multitemporal imagery. This sequential Landsat series
is the first and the last set available for this area
between 1972 and today with such short intervals.
95 releves following standard ITC procedures (Gils
et.al. 1984) were available as field reference
Photo-interpretation followed the landscape-guided
method (Gils et.al. 1984).
Pre-processing of Landsat CCT's started with radio-
metric corrections (sun angle, haze) and producing
square pixels using standard IPL - ITC methods (Mul
der 1982). The geographical correction could not be
carried out by routine procedures due to lack of
topographical orientation points in the survey area.
Therefore the three Landsat images have been super
imposed visually. The normalised vegetation index
IR-R is calculated for each of the three sequential
IR+R scenes.
Each of the three temperorally different vegetation
indices has been coded by a colour. The vegetation
index values have been scaled from zero to hundred.
The highest to the lowest vegetation index in January
has been assigned a corresponding hue in red.
Similarly the vegetation indices in May are coded in
green and those of the October image in blue. The
three seasons superimposed produce a coloured map on
approximate scale 1:250 000. For details and back
ground see Mulder (1982).
The scene of October 1979 has been subjected -
after standard correction (compare under sequential
imagery) - to a supervised classification (SC) with
the help of the field data. 141 Pixels were located
on the scene were the land units were known. The Red
(x-axis) and Infrared (y-axis) radiation intensities
of the 141 samples were plotted in a feature space.
The 141 were classified first into their land unit.
A cluster analysis was performed and regions were
delineated. Each delineated region in the feature
space was colour coded. Thereafter all the pixels
were given a colour according their place in the
classified feature space.
The land units are named in the legend according to
their vegetation, since this is their main charac
teristic to be observed both on the image and on the
ground, due to the flatness of the area and scarcety
of crops and artifacts.
Result and discussion
The supervised classification of the Landsat image
resulted in 8 main legend units implying also 8 main
vegetation legend units. The legend units are repre
sented in the legend of figure 1. The land units
might be compared with those obtained with other
image processing techniques of Landsat data as there
are the false colour composite used by the Mefit-
Babtie (1983) survey and the multitemporal image of
the present paper.
The standard Landsat image plus aerial photography
interpretation by the Mefit-Babtie (1983) team re
sulted for the same area covered by the present paper
in five vegetation legend units. There is more
differentation in the Toich area with the supervised
classification of the Landsat data as compared with
the map based on standard Landsat products. However,
the lower resolution of the standard might be an
artifact, because the map from which this conclusion
derives is on a scale 1:500 000 and the variety
within the Toich could possibly not be mapped on this
scale, but is cartographically representable on the
250 000 scale of the supervised classification.
The field sampling - on which the supervised
classification was based - was designed in the hypo
thesis of an east-west catena of land (including
vegetation) units. This catena is well expressed
both in the image (fig.l and fig.2) and the Mefit-
Babtie map (1983). However, the images (fig.l and
fig.2) show a north-south catena in the toich form
high (dry) to low (wet) sofar not noticed. This
north-south catena within the Toich still has to be
confirmed by field observations.
The multitemporal image (fig.2) has basically the
same land unit pattern as compared with the super
vised classification. The multitemporal image (fig.2)