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instrument design uas therefore net yet available. Consequently, the
task now uas to remedy the kneun limitations and shortcomings of the
Stereomat system in the shortest time and uith the least effort
possible.
The first functioning Stereomat uas constructed using a Kelsh-type
projection instrument. This resulted in the follouing principal
technical difficulties:
1. To assure correlation of identical picture details even uith
very lou contrast, the signal-to-noise ratio at the output of
the scanning photocell must be as high as passible. Because of
the very small relative aperture of the projection optics,
necessary for depth of focus, and because of the limited bright
ness of the light spot on the cathode ray tube screen, the signal-
to-noise ratio is extremely unfavorable and cannot be improved.
2. Uith the design used in the projection-type instrument, the
screen of the cathode ray tube is horizontal and does not follou
the model slope. As a consequence, if the slopes are pronounced
the terrain areas scanned are different in each of the tuo
pictures. This results in an intolerable falsification of height
measurement in terrain of more than 20° slope. If separate cathode
ray tubes are used for the left and right pictures, the scanning
pattern in each picture can be corrected according to slope, and
such height errors thereby eliminated.
3. The inertia of the carriage system limits the speed of model
scanning. To reduce the time required for the scanning of a picture
pair, the dimensions of the stereo model should be as small as
possible. According to Hobrough the difficulties caused by carriage
system inertia increase at least uith the third pouer of the linear
dimensions, uhile the effort necessary to maintain constant re
lative measuring accuracy uith diminishing model scale increases
only at a linear rate and is thus uithout significance.
From these considerations it follous that the possibilities inherent
in the Stereomat system can be exploited only uith a restitution instru
ment having separate scanning optics of large aperture for both pictures,
and uith relatively small stereo model dimensions.