2 
scanned mechanically by two multiplier phototubes . The tubes were 
mounted on a scanning platen which traveled the full width of the stereo 
model in the Y-direction and was shifted 1/ 32 in. in the X-direction at 
the completion of each Y traverse. The wave forms of each diapositive 
during a line scan/ were subtracted from each other and the difference 
was integrated. The assumption was made that when the two signals 
were nearly identical/ the difference between them would be close to 
zero. The equipment was able to produce very wide contour bands. It 
was the first use of a type of subtractive correlation circuit to determine 
the trend of a number of adjacent image points. 
Work on a projection type stereoplotter/ and a nipkow disk- 
multiplier phototube scanner in place of the tracing table was continued 
under the aegis of the U. S. Army Engineer Research and Development 
Laboratories. Eventually electronic circuitry performing a mathematical 
correlation between the two video signals was developed and used (3). 
Useful contours and orthophotos were printed. The equipment, however, 
still did not compete with a manual operator, time-wise or economically. 
In 1956, two companies, working independently, each began 
development of an automatic contour plotter. Both used flying spot scan 
ners in place of the tracing table, and both used electronic correlation 
circuitry to determine when the face of the flying spot scanner tube was 
on the surface of the stereomodel. One plotter, the Stereomat, developed 
by G. L. Hobrough, then with Hunting Associates, Limited, Toronto, 
Canada, demonstrated that contour manuscripts with C factors of over five 
hundred could be drawn at speeds somewhat faster than human operators (4) . 
The other plotter development was funded and directed by Rome Air Develop 
ment Center. The plotter was designed and constructed when the authors 
were in the employ of Librascope Division, General Precision, Incorporated, 
Glendale, California. 
The R.A.D.C. plotter, officially called Stereoplotter, Projection 
AP-14, consists of a projection type stereoplotter, a photoscan system and a 
correlator system. Additional subsystems are provided to integrate the func 
tions of the major systems. Use of the instrument's manual and electronic 
tracing aids enable an operator to employ it as a conventional stereoplotter.