Time Required for the Various Operations 
The participants were requested to supply data on the time used in 
various orthophoto operations. Since this experiment was based on only one 
single stereomodel, the information available is not too significant. 
However, some general conclusions on the time involved can be drawn. 
The initial photogrammetric operation, including the preparation 
of the control manuscript and the relative and absolute orientation 
procedures, should not differ significantly for the various analog instru- 
ments used; it should be approximately the same as for conventional photo- 
grammetric plotting operations. An average of 1.5 hrs. was reported by the 
participants for this operation. For the Gestalt Photo Mapper, which is 
based on an analytical relative and absolute orientation procedure 
supported by the automatic image correlation, the combined time required 
for preparation and orientation was 37 minutes. 
The time required for the profiling procedure depends, among 
other things, upon the profile density, the terrain characteristics, the 
scanning speed selected by the operator, and, for certain instruments, on 
the enlargement factor used for the orthophoto production. Most instru- 
ments offer the opportunity to vary the scanning speed. Expressed at the 
scale of the photographs, these ranged from 1.0 mm/sec to 2.0 mm/sec. 
The average scanning speed was 1.6 mm/sec. A slit length of 5 mm at 
orthophoto scale and 4x enlargement would correspond with a slit length 
of 1.25 mm in the original photo. Assuming a 60% longitudinal and 30% 
side overlap, the average scanning time for a stereomodel would be 2 hours, 
not counting the time for occasional rests which, we think, cannot be 
avoided. 
For instruments in which slope corrections normal to the profile 
direction are applied, a larger slit size can be used, resulting in a 
significant reduction of scanning time. This is demonstrated for the 
Wild OR 1 (slit in photo scale 2 mm) and Zeiss GZ-1 (slit in photo scale 
2.4 mm) for which the times reported for the manual profiling operation 
were 1 hr. 20 min. and 1 hr. 45 min., respectively. The use of automatic 
image correlation results in a further time reduction. For instance, the 
time reported for the Gestalt Photo Mapper was 1 hour to produce each, 
orthophoto together with simultaneously recorded and plotted height 
information. 
It is interesting tonote that in this experiment the time needed 
to scan the model in a profiling mode is considerably shorter than the 
time needed for contouring. It took an experienced operator 8 hours to 
produce the pencil-drawn contours of the Ripon test area on the Wild A7 
(Fig. 2) as compared to an average of approximately 3 hours reported for 
the manual profiling of the same model. Obviously the question of 
accuracy should also be considered when comparing both approaches. 
The scanning speeds in off-line orthophoto projectors which use