Fig. 3 Contours plotted from droplines 
A contour map for the same area obtained with the dropline technique 
used in the Gigas Zeiss orthophoto system, is shown in Figure 3 for 
comparison. The superior quality of the stereo-orthophoto contours 
with respect to terrain detail is obvious. 
The vertical accuracy obtained with the crude experimental orthophoto 
equipment and the prototype Stereocompiler was found to be 0.3 9/oo 
of H for spot elevations to 0.5 9/oo of H for contours. In the pre- 
sentation of detail, the stereo-orthophoto contour lines have demon- 
strated their superiority over contours obtained by dropline techniques. 
The main error source affecting the vertical terrain information de- 
-. xáved from the mapping experiments described in this paper are no 
doubt due to the two separate profiling operations of the orthophoto 
and the stereomate. Since profiling errors have very little effect 
on the vertical stereo-orthophoto accuracy when the orthophoto and 
stereomate are based on identical profiles, considerably improved 
results can be expected when properly designed equipment is used. It 
is interesting to note that then a correct height can be derived for 
a certain terrain feature, despite the fact that such feature may be 
subject to a planimetric error caused by approximations in the recti- 
fication process or a profiling error. 
The plotting operations performed in the Stereo-compiler are related 
directly to the orthophoto image. The planimetric accuracy offered 
by the stereo-orthophoto technique is therefore in principle the same 
“13 
  
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