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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.