in the system's tape storage unit, Since adjacent profiles may 
be expected to be quite similar, the final tape from one profile 
is used as the initial control for the next, and the operator 
merely exercises his override option to produce the correct 
profile. 
Another important feature of this instrument goes into 
operation when the final profile is obtained. During the 
memory aided scanning, the tracing table platen can be inter- 
changed with a miniature Vidicon tube which is the sensing 
element of a closed circuit television system used for pro- 
ducing orthophotographs. 
2. The Orthophotoscope. This is essentially an xy 
coordinatograph under a light tight cover. Its motion for each 
profile is controlled by the final magnetic tape produced in 
the stereoplotter. It positions a high intensity cathode ray 
tube which exposes on a sheet of film the image being viewed by 
the Vidicon of the closed circuit television system attached to 
the tracing table in the plotter. Since the Vidicon is driven 
in the z direction according to the terrain elevation, the 
image which it receives is automatically corrected for scale. 
Therefore the photographic record produced is a true ortho- 
graphic projection and all planimetric and topographic features 
appear in their correct position. The orthophotograph is 
therefore a highly acceptable substitute for the usual map. 
By interchanging gears in the drive mechanism and 
inserting lenses between the cathode ray tube and the film, it 
is possible to change scale between the stereoplotter and the 
orthophotoscope. 
3. The Line Drop Plotter. This unit provides information 
for plotting contour lines. At the moment it is probably more 
accurate than the Stereomat, but when perfected, the Stereomat 
should be more efficient. 
It is apparent that the combination of the automatic 
orientation, contouring, and profiling, provided by the 
Stereomat, with the digital profile and the orthophotograph 
produced by the Integrated Mapping System would eliminate the 
remaining bottlenecks in the photogrammetric-electronic highway 
design system. When these devices are perfected, they will 
doubtless be incorporated into the system. 
There is one further development which eventually may 
Prove more powerful than any so far described. Professor 
C. L. Miller at Massachusetts Institute of Technology has been 
experimenting with what he calls a Digital Terrain Model. His 
research has been sponsored by the Massachusetts Department of 
Public Works and the U. S. Bureau of Public Roads. This 
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