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obtained. Contours are being detected and traced on stereomodel areas 
having slopes that lie in a range between 6 and 60 degrees. The ability 
of the automatic search system to detect contours which exist in each 
stereomodel area is both rapid and positive. 
Figure 7 shows a photo of the display tube in which several 
contours were plotted automatically with the contour interval being 1/1000 
of the flying height. The correlation circuitry has no trouble in keeping 
the scanning beam on the proper contour. Actually, in the laboratory, 
under controlled conditions of diapositive image density content and con 
trast, contours can be plotted where the contour interval is 1/3000 of the 
flying height. 
Figure 8 is a check for plotting repeatability. The same contour 
has been traced automatically fifteen times. Repeatability is at least as 
good as a human operator. 
Figure 9 shows a comparison of the automatic contour output of 
the Stereoplotter, Projection AP-14, versus the same terrain contoured 
manually by an experienced stereoplotter operator. The stereomodel is 
being contoured properly by the electronic system,with a majority of the 
contours being filled in. The unfilled contour lines occur in areas where 
the contour-following servo loop is having difficulty in obtaining an error 
signal as to which way to go. This occurs when moving along a contour 
from a region of good correlation to a region of poor correlation. More 
work on the contour following circuitry is needed in this area. 
As is common with all instruments which employ electronic scan 
ning of stereo diapositives, performance is very intimately associated 
with the particular type of detail contained in the diapositive areas of the 
raster scan. In particular, the scanning of diapositive areas containing 
very little detail, or detail consisting of a single gray shade, result in 
poor instrument performance. This degraded performance is directly re 
lated to the decrease in the signal-to-noise ratio of the resultant video 
information, with a resulting loss of steering signals for the raster servo 
system. The use of diapositives which have undergone electronic dodging 
gives the best instrument performance.