tion by the desired contour interval. The operating 
point moves up (or down) the terrain slope to the 
next contour, as indicated in Figure 9. When indexing 
at a boundary, the system moves (approximately) 
along the boundary to the next contour line. When 
indexing on contour closure, the system moves di- 
rectly up the slope from the closure point to the next 
contour. Indexing in profiling involves moving to a 
parallel profiling line at a specified distance from the 
profiling line just completed. 
U-- ---4 
Model or Photo 
Boundary 
  
Indexing at Boundary 
Start 
Indexing on Contour Closure 
Figure 9 Automatic Indexing During Contouring 
Automatic indexing between parallel profiling lines 
can occur in this way until the entire area has been 
profiled. Automatic indexing between contours can 
occur in this simple way until the top of a hill (or 
bottom of a valley) is reached. Contouring is nor- 
mally started at the lowest (or highest) contour eleva- 
tion in the model. At each contour elevation, only 
the connected part of the contour line will be plotted 
automatically; disconnected sections must be plotted 
separately. When a saddle point on a hill is reached, 
the system will automatically continue contouring up 
one peak and ignore the others. Upon reaching the 
top of each peak in the stereomodel, the operator 
must (in present systems) stop automatic plotting, 
locate the lowest unplotted contour around another 
peak, and restart automatic plotting. 
Automatic plotting can proceed along successive 
lines as described above as long as terrain image con- 
ditions are good. If image conditions become poor, 
the terrain surface may be lost by the stereopercep- 
tion system. When the terrain surface is lost, auto- 
matic plotting must be stopped to avoid plotting of 
grossly incorrect data. Detection of lost situations 
during automatic plotting is performed primarily on 
the basis of low measured correlation. Lost condi- 
tions occur most often in areas which lack terrain 
detail, such as lakes and large featureless fields. Lost 
conditions may also occur when the terrain slope 
changes suddenly, such as in crossing the top of a 
sharp ridge, particularly if there is a large shadow area 
associated with the ridge. 
The computer can be programmed to attempt 
automatic recovery from lost conditions by perform- 
ing a systematic search. A three-step automatic search 
strategy, which has proved to be highly effective in 
the automated analytical stereoplotters, is: (1) lift the 
stylus and stop the plotting to allow parallax and 
slope measurement errors to be corrected, (2) make a 
series of small trial motions based on possible changes 
in plotting line direction, and (3) move forward in the 
original direction (at constant elevation for profiling) 
for a larger distance. The first or second steps are 
generally quite successful in recovering from lost 
conditions associated with sharp ridges or valleys 
without generating a large gap. The third step is most 
useful for bridging rivers and lakes in the profiling 
mode. In some situations, of course, the search 
strategy fails, and the computer must then signal the 
operator for manual assistance. 
AUTOMATIC PLOTTING PERFORMANCE 
A typical contour manuscript, automatically plot- 
ted by an automated analytical stereoplotter, is 
shown in Figure 10. This chart was plotted from 
vertical photography at a scale of approximately 
1:48,000. The flying height was 24,000 feet. Within 
the area shown, three types of terrain geometry 
occur: rolling hills in the upper portion, a flat region 
at the central right, and a more sharply mountainous 
region at the lower left. 
In the rolling hills, the system plots rapidly—at an 
estimated three to five times the speed of a human 
operator. The occurrence of transient errors and lost 
conditions is negligible, and the automatically com- 
piled manuscript requires almost no editing. 
In the flat areas, the system plots less rapidly and 
generates more gaps, due both to poorer image detail 
in the open fields and to transient elevation errors 
which accompany the parallax-integral steering. 
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