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

3. The third one concerns the repartition of the vegetation.
Studying the significance of the limits, we were led to
distinguish the limits which are related to the superficial for
mations, among the others limits which reflect the vegetal
competition between species, or the way the ground has been
used by man. The two figures below show the vegetal limits
related to structural limit of superficial formations (overmarked
figure 2) and the spatial coincidence between these limits
(overmarked figure 3), and the structural limits (figure 1).
.1 ___
Figure 2 : Soil and vegetation sequences
Therefore it seems possible to express in the aerial
photography, the proper information on the structure of the
whole volume of superficial formations, by the intermediary of
the surface différenciation and the vegetal cover repartition.
1.3. The methodology defined for image processing
The first group of treatments applied to the images, were seg
mentation on the plane of the reflectances, by establishing
different classes of vegetation, in one dimension (primary
channels, channels combined by vegetation index) and in two
dimensions (two first axes of the P.C.A.). The results of these
treatments were confronted with the ground reference and
thereby we conclude that this kind of treatments do not permit
to distinguish the most important vegetal domains, for they are
formed by the same associations of vegetal classes.
This first observation lead us to perform a sequence of
treatments based on textural features. Textural images of
gradient (Sobel) were calculated, a local density of gradient was
extracted by the convolution with a 7x7 scanning window, and
this textural information was mixed with the reflectance one ,
on the same level, in a Bayesian classification. The choice of
that sequence of treaments was conducted by the reflection that
natural photography is interpreted by human vision, putting a
hierarchy upon limits, the strong one upon reflectance limits,
and at an inferior level upon the textural ones (reflectance ar
Now, we saw that the main limits from the point of view of
structural study of superficial formations were separating
domains with the same associations of vegetal classes, giving
the same visual impression of colour, but with different arran
gement of classes which give a visual impression of different
textures. The way of expressing textural information at the
same level that reflectance one, makes images where the im
portant limits are very visible, and which are very near to the
map of vegetation repartition we obtained both by
interpretation and by field s.udy. The two figures below are
showing the interpretation of the natural photography (which
can be compared with the map of vegetation, figure 3), and the
image resulting of the sequence of treatments described. We
can see that this image is nearer from the map than is the
interpretation of natural photography.
Figure 4 : Photo-interpretation of the natural photography
Figure 5 : Image resulting of ¡he studied sequence of treatment
Fig. 1 shows the site under study. There, the granitic shield is
covered with a clay-sandy material ('Sérié Détritique de Base'
for geologists). Tropical forest covers the whole shield (more
than 97% of this country area.)
Studies made in Guyana by BOULET and al., starting from
1976 show that the initial ferrallitic cover, developped from the
weathered shield, is now out of equilibrium and is therefore
being transformed. The initial ferrallitic soil is tranformed here
into podzol, and in geographical space, this transformation is
more or less rapid; these transformations act at the scale of the
existing topography, that is here about 1 km; and the transition
between the ferrallitic pole and the podzolic one may be very
short (100 m.) or may cover the whole watershed.