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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.