854 
area with a coverage of tropical rainforest it was found that drainage information 
obtained from stereo radar interpretation was quantitatively better than that shown 
on the 1 : 25, 000 topographic maps of the area. Qualitatively the interpretation 
was also reliable, with exception of the erroneous connection at 1 (fig. 11). This 
cannot be said of the monoscopic radar interpretation, where the linking of 
stream channels was often problematic. The topography of the area is structurally 
controlled; the well developed strike ridges strongly influenced the monoscopic 
interpretation of the drainage network. Compared with the topographic map the 
ratios are for mean drainage density: 
D (radar stereosc.) = 1.18 D (map) 
D ( radar monosc.) = 0. 76 D (map) 
D (aerial photo) = 0. 88 D (map) 
To analyze the influence of relief on drainage interpretation from radar images, 
several more basins with different relief aspects have been studied: 
a. High relief areas (local relief differences in the order of 1000 metres or more). 
b. Low relief areas (local relief differences up to 100 metres), 
d. Flat terrain. 
In addition, vegetation differences have to be taken into consideration. A dense 
tropical vegetation influences the interpretability of the radar image. Water sur 
faces of small rivers are not visible due to the continuous cover of the forest 
canopy. Moreover, small valley depressions are less conspicuous. What is ob 
served is the upper canopy of the tropical forest, which smoothes out low relief 
differences, as larger trees usually grow in the lower areas. In the Ka and X 
band radar, the penetration capability through tropical rainforest may be neglected. 
Delineation of water channels in flat areas has to be carried out on the basis of 
slight depressions in the forest canopy, vegetational changes along the river 
courses (alignment of palm trees along riverlets is often found in Colombia), or 
by water surface reflection where small open areas occur. 
With image interpretation of single strips use will be made of radar shadow, which 
will give the interpreter a relief impression. Topographic slopes, dipping towards 
the flightline, will reflect a lot of energy, when the incidence angle of the radar 
o 
beam approaches 90 . Such zones appear very light in tone on the image; shadowing 
is minimum and the ’’blooming" effect will suppress small tonal differences, other 
wise visible. Consequently relief expression is minimal. 
On the other hand slopes dipping away from the radar might fall entirely within 
the zone of radar shadow when the angle of topographic slope is larger than the