856 
will mentally obliterate the information seen with the other eye. By alternatively 
viewing with one and two eyes through a stereoscope, the monos copic information 
from the shadow zones might be seen in the general three dimensional context 
of the surroundings. 
The drainage network interpreted monoscopically and stereoscopically does not 
differ too much in this high relief area. Certain zones show less detail on the 
monoscopic image as a result of the aforementioned phenomena. Monoscopically 
some drainage channels are erroneously linked and in one point the basin area 
was incorrectly delineated. This is mainly a result of not seeing true relative 
altitude differences. 
3. 3 Medium to low relief areas 
In the low relief areas drainage patterns interpreted from single strip imagery 
differ considerably from those delineated from stereo radar images. In general 
it was found that drainage density, stream segment number, and stream length 
were higher for the stereo interpretation than for monoscopic interpretation 
(table 1). The checked drainage basins were situated in a semi-dry tropical area 
(Atlantico, Colombia, Ka and X band radar) and in a wet tropical forest area 
(Choco, Colombia, Ka band radar). 
In several instances stereo interpretation provided a higher drainage density 
than the available standard topographic maps 1 : 100, 000 and 1 : 25, 000. The 
similarity between the drainage network of radar stereo interpretation and a 
topographic map is generally higher than that of single strip interpretation and 
a topographic map. Especially well defined narrow basins with a relative high 
"basinshape" = basmleagil!. ( 
A basin area 
do show a higher similarity between monoscopic and stereoscopic radar inter 
pretation. 
For basins with a low ratio the qualitative drainage interpretation of single 
strips is far less reliable. This is especially true for slightly hilly terrain of 
uniform height, where the valleys are not sharply defined due to little incision 
of the streams or due to forest cover. Linking of stream channels on single strip 
imagery might be more or less at random. Stereoscopy is in these cases indis 
pensable. 
3.4 Flat areas 
For flat areas where altitude differences are in the order of a few metres, 
stereoscopy does not help very much. In non-vegetated areas tonal differences 
Table 1