pt "- UNW 
for two or three millimeters at photo scale. Alterna- 
tively, the rough data might be recorded on magnetic 
tape and be processed at a later time or by another 
computer. 
Prescanning and storage of stereomodel data offers 
a powerful approach to removing many of the uncer- 
tainties encountered in automatic plotting, particu- 
larly in automatic contouring. In concept, the stereo- 
model is prescanned in a parallel profile mode, 
developing a matrix of terrain elevation measure- 
ments which are stored for later use in guiding the 
detailed plotting. Simultaneously, data indicating the 
performance of the correlation system (and para- 
meters sensed by the pattern-recognition system, if 
available) are stored for later use in optimizing the 
scan-size and plotting-speed strategies. During the 
detailed plotting, the stored information is used to 
compute the approximate path of each contour, and 
this information guides the automatic plotting pro- 
grams in following the detailed contour. With this 
guidance, the plotting can proceed at a much higher 
speed. Flat open areas where marginal image detail 
normally occurs can be plotted with maximum scan 
size. It would probably still be necessary to plot 
slowly in some mountainous areas, but the portions 
which must be plotted carefully are preidentified, and 
the distances plotted in this mode can thus be mini- 
mized. 
One method of implementing the prescanning 
operation is to combine it with another compilation 
task such as orthophoto generation, which inherently 
requires parallel profiling of the entire stereomodel. 
However, as discussed in an accompanying paper,* 
*J. J. Edmond, “Orthophoto Generation.” 
AUTOMATION IN COMPILATION 
there are also substantial advantages in prescanning 
and off-line compilation of the orthophoto. 
Many other possibilities exist for expanding the 
computer functions. In practice, the potential in this 
area is limited only by the computing capability of 
the particular machine. Even here, the techniques 
cited primarily increase the computer memory re- 
quirements, an area where most machines are readily 
expandable. 
CONCLUSIONS 
The combination of image correlation techniques 
and stored-program digital control available in the 
automated analytical stereoplotters provides a power- 
ful approach for automatic stereocompilation. In 
present instruments, use of these techniques relieves 
the operator of the stereoperception and plotting 
tasks in most routine situations. With further re- 
search, it can be expected that the capabilities of 
present instruments will be expanded to accommo- 
date the most difficult terrain and photographic 
conditions. Beyond this, further application of digital 
data processing techniques will further improve the 
efficiency of photogrammetric compilation. 
REFERENCES 
1. G. L. Hobrough, “Automatic Stereoplotting,”” Photogrammetric 
Engineering (1959) pp. 763-769. 
2. G. L. Hobrough, “Automatic Stereo,” Report to the IXth Interna- 
tional Congress of Photogrammetry, London, 1960. 
13