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extracting wide-band electrical signals from conjugate diapositive areas. 
Use of a single scanning source assures perfect alignment of the scanning 
rasters on each diapositive. The scanning raster is composed of a geo 
metrical rosette pattern with a high-frequency X-axis wobble superim 
posed on it to enhance differential parallax detection. The rosette pattern 
occupies a small portion of the total scanner face area and can be posi 
tioned to any area of the scanner's face by either manual or correlator con 
trol. A beam-intensity feedback system maintains the scanning spot at a 
constant brightness level. 
The correlation system includes the necessary electronic circuits 
to extract information regarding the scan pattern's position in model space. 
The correlators utilize the information derived by the photoscan system from 
the diapositives and sense model parameters—such as height error, slope 
vector, and information density—in each stereomodel area occupied by the 
scan pattern. Feedback circuits, utilizing the detected model parameters, 
cause the scan pattern to automatically locate and follow any contour that 
exists within the model area intercepted by the scanner. 
In its present configuration, diapositive orientation is performed 
manually, as are the tasks of scaling and leveling of the stereomodel. The 
scanning cathode ray tube intercepts a portion of the neat model and auto 
matically extracts contour information for that portion of the stereomodel. 
After completing a section of the neat model, the scanner is manually located 
to the next section. The final map manuscript is compiled by connecting 
each of the map sections together and observing their proper coordinate 
location. 
Contours are initially acquired by means of a preprogrammed search 
pattern which causes the rosette scan raster to sequentially move from the 
center to the edges of the scanner in a series of straight-line search sweeps. 
The operational mode is changed from search to contour following by con 
tinuously monitoring the level of video signal cross-correlation. When the 
cross-correlation voltage exceeds a preset level, indicative of no height 
error existing between the scan raster level and the stereomodel surface, 
the operational mode required for automatic contour following is activated; 
and the contour is traced to the edges of the scanner cathode-ray tube.