a N SE EM 
The Chotts (named also Playas or Sebkhas) are "flat floored bottom of 
an interior desert basin on which smooth, barren, sun baked microrelief 
prevails" (Chico, 1968) - The Chott Merouane and the Chott Melrhir present a 
minimum elevation of 40 meters below sea level and their bottom is salty and 
may be filled with a sheet of water in winter. They have been in existence 
since the beginning of the Quaternary and are located on a subsiding area 
(Ballais, 1982) 
This area is of interest because of: (1) the absence of relief, (2) 
the well delineated interface between Chotts and sand, and (3) the different 
backscattering of the Chott floor due to the fact that Seasat and SIR-A data 
were not taken at the same time of the year or due to characteristics of the 
radar systems. 
DATA PROCESSING AND FIRST RESULTS 
SIR-A was data were scanned with a microdensitometer (pixel size: 50 
pm, digitized (recorded on 256 gray levels), and registered to the Seasat 
data (4800 lines and 2400 samples) using JPL's VICAR image processing 
programs. The registration process consists of finding common tie points for 
rotating and sizing one image (SIR-A) to another one (Seasat). About 120 tie 
points were used at full resolution. The angle between the two paths was 939, 
but in the process the necessary rotation was only 39 (due to the way the SIR- 
A image was scanned), so we have a radar picture over an area taken by two 
sensors whose look directions were almost perpendicular. 
Registration of multispectral data have been done recently in order 
to enhance: (1) rock type discrimination (Blom, 1981; Evans, 1982 - this 
issue), (2) influence of radar illumination directions and surface roughness 
properties (Elachi, 1982) and (3) SAR rectifications (Ulbricht, 1980). 
The final product (Fig. 3) is an image at the same scale emphasizing 
the role of each imaging radar system. Over this picture, we choose three 
sub-areas displayed at full resolution in Figures 2a, 2b and 2c. 
INTERPRETATION OF FIGURE 2 
Figure 2 displays the Chott Merouane-Chott Melrhir area as seen by 
Seasat (top) and SIR-A (bottom). From the left to the right are the Chott 
Merouane (B-C, 2-5) and the Chott Melrhir (D-H,l-4). Those salty depressions 
are surrounded by Quaternary deposits (A-B, 1-5); D-F, 2-4) or by dunes (F- 
K,5). Running North South from the Aures Mountains several channels bring 
alluvium in the Chott Melrhir; some of them are seen in the upper right corner 
of the figure (J-K, 1). Even though the climate is very arid, some places are 
inhabited as the Bordj Stile (A,1), the Bordjs Ouvrir and Mrhaier (A-B,3), the 
Bordj Krelil (A,5) and the Bordj El Hamraia. These villages are connected by 
small roads (A, 1-2) or railroads (A-B, 1-2). 
There are few changes in the shape of the objects on the two pictures 
which were taken 3 years a part. But, a small Chott at (E-F, 5) seems to have 
larger boundaries on SIR-A data suggesting a little expandion over the 
years. The reason or the process of this expansion is not known. 
The sandy areas (F-J, 5; A, 1-5) look the same over the two pictures 
and have a coarse texture. On the left side of the picture, the area presents 
some lineations with little vegetation. 
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