Plotting Scale — 1:24,000 
(Reduced to 1:48,000 here) 
Contour Interval — — 50 ft 
Flying Height — 24,000 ft 
Figure 10 Typical Contour Chart 
However, these gaps present no problem; since they 
are generally much smaller than the contour separa- 
tion, they can be easily edited. The reduction in plot- 
ting speed is also relatively unimportant, since the 
time spent in plotting the flat regions is a minor frac- 
tion of the overall contouring time. 
The system experiences its greatest difficulties in 
the mountainous terrain. The plotting speed is lower 
than in the rolling hills, and a substantial number of 
gaps are generated by raising the stylus to avoid plot- 
ting errors in steering around sharply breaking fea- 
tures. The significance of the gaps is much greater in 
this situation, of course, due to the higher slope of 
the terrain. Thus, manual fill-in of these gaps is gen- 
erally required. However, even in mountainous ter- 
rain, automatic contouring generally provides a sub- 
stantial saving in time over manual operation. 
In Figure 10, the contour interval (the vertical 
spacing between contours) is 50 feet. In Figure 11, 
the contour interval has been reduced to 12.5 feet, 
1/2,000 of the flying height, in plotting a small sec- 
tion of the stereomodel. This chart demonstrates that 
the precision of the automatic plotting is very high. 
Comparisons of plotted lines with careful manual 
elevation measurements have also shown that the 
accuracy is high. Tests of the AS-11B automatic ana- 
lytical stereoplotter over a wide range of photo- 
graphic formats, scales, and terrain types indicates 
AUTOMATION IN COMPILATION 
that the average elevation error associated with auto- 
matic contouring is in the range 0.015 to 0.030 milli- 
meter rms at photo scale. This accuracy is equivalent 
to that achieved in manual operation of the analytical 
stereoplotters. 
The high accuracy available in the automatic con- 
touring with these instruments indirectly emphasizes 
a basic limitation of present-day automatic plotting 
systems: they do not correct for the heights of trees 
and other obstacles encountered on the terrain sur- 
face. For this reason, automatic plotting presently 
finds its major applications for small-scale and 
medium-scale mapping, at scales of 1:50,000 or 
smaller. (An exception to this statement is the use of 
automatic plotting for orthophoto compilation, 
where it is desirable to plot the visible surface rather 
than the terrain surface.) 
AREAS OF POTENTIAL IMPROVEMENT 
As illustrated by the preceding discussion, the 
central problems of providing automatic stereoper- 
ception and automatic plotting for the analytical 
stereoplotters have been solved. The presently de- 
veloped correlation equipment and computer pro- 
grams provide substantial improvements over manual 
contouring and profiling for a wide range of photo- 
graph scales and terrain-geometry conditions. There 
are, however, a number of situations where the per- 
formance of the present systems is marginal, and 
where they require a substantial degree of assistance 
from the operator. Generally, the automatic system 
fails in these situations because the scanning and cor- 
relation process does not derive enough information 
from the imagery to provide adequate steering signals. 
Plotting Scale — 1:12,000 
Contour Interval — 12.5 ft 
Flying Height — 24,000 ft 
  
Figure 11 High-Resolution Contour Chart