reshape the contours and significantly improve the over-all 
accuracy of the contour plot. Experiments conducted on several 
contour plots indicated that, after reshaping or editing the contours, 
a "C" factor of 850 could be obtained. 
f. The horizental accuracies of the orthophoto output 
were evaluated by comparing the position of identifiable features 
on the orthophotograph with those of published maps of the same 
area. Numerous orthophotographs were prepared and evaluated with 
published 1/25,000 scale AMS and 1/24,000 scale USGS topographic 
maps, photographically enlarged to the scale of the orthophoto. 
Over 90% of the linear detail of the orthophoto, except where 
there was reason to believe the map features had been carto- 
graphically displaced in the topo map, were within 0.02 inch of 
their true position at map scales. A partial model of an ortho- 
photograph produced with the Stereomat is shown in Figure 4 and 
the unretouched contours superimposed on this orthophotograph 
are shown in Figure 5. 6 e 
g In addition to the single model tests of the horizontal 
accuracy of the orthophoto, a test of the horizontal accuracy 
of an assembly of orthophotographs was evaluated.  Orthophotographs 
of 15 terrain models, covering a 1/50,000 scale quad, were prepared. 
The orthophotos, at a scale of 1/25,000, were reduced photographically 
using a single reduction camera setting to reproduction scale. 
Horizontal control, which had been identified initially on the 
diapositives with a Zeiss Snap Marker, an identifying mark which 
reproduced on the orthophoto, and the plotted control points on 
the format board, were used to control the mosaic of orthophotos. 
The orthophoto mosaic was then compared to the published map. 
The horizontal accuracy of the orthophoto met the requirements 
of the National Standards of Map Accuracy. 
h. During the test operations, time records were kept of 
all operations. The fully automatic relative orientation mode 
could not be used in the grid model flatness tests. However, 
the relative orientation of the grid model using the manually 0 v 
controlled correlation circuits was accomplished in 12 minutes. 
i. The time required for relative orientation of a stereo- 
model is a function of the relief in the model, and varied between 
6 and 10 minutes for an average of 7.5 minutes. Absolute orientation 
required approximately 15 minutes per model. 
j. The time required to produce test orthophotographs 
ranged from 1.7 to 2.6 hours per model, with an over-all average 
of 2 hours per model. The time required to produce an orthophoto- 
graph is dependent upon terrain and quality of the aerial photography. 
k. From the contouring accuracy tests, it was determined 
that the average time required to contour a stereomodel, at a 
contour interval suitable for 1/50,000 scale map production, was 
8 hours. Contour compilation times varied from 6 to 12 hours per 
model, and varied as a function of relief. Operator intervention 
was very high in the contour mode, while it was minimal in the 
orthophoto mode. 
18