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DIFFERENTIAL RECTIFIER SYSTEM 
A system is envisioned which would produce planimetrically-corrected photo- 
graphic images (orthophotos and stereomates) or other simulated-perspective 
reconstructions by the digital differential rectification of aerial or 
terrestrial photograph (Fig. 9). This system would be based on the scanning 
and playback capabilities of the Perkin-Elmer PDS product line of Photodigit- 
izer Data Acquisition Systems. 
The heart of the System would be the Differential Scanner. It would include 
an X-Y stage assembly which could carry a 9%-inch aerial photograph for 
scanning, or the raw film stock for playback. Its servo-controlled motors 
would drive the photo carriage at a scan speed of up to 200mm per second, 
while high-resolution linear encoders would monitor the stage position to 
within 5 micrometers of true position. 
Beneath the stage would be a tungsten-halogen light source which would be 
used to uniformly illuminate the patch of film being scanned by transmitting 
light through the photographic transparency. The illuminator assembly 
would also contain the LED and pixel-defining apertures which would be used 
to expose film during the playback mode of operation. Above the stage would 
be the image dissector and related electronics which would provide the non- 
orthogonal scanning capability required of this system. Here, too, would be 
the sampling and digitizing electronics needed to transform the optical 
information of the photo into digital data. 
The basic Differential Rectifier System would include a suitable computer; 
a dual magtape cartridge drive for program input and data input storage; a 
24-line CRT keyboard terminal with hardcopy attachment; and two 1600 cpi, 
45 ips, phase-encoded magtape drives. 
SUMMARY 
A method has been described for producing planimetrically-corrected recon- 
structions from central projection photographs by a process of digital 
differential rectification involving the non-orthogonal scanning of the 
photo followed by the orthogonal playback of the digitized data. A piece 
of equipment suitable for performing this differential rectification has 
been described. The advantages of this method would be high geometric 
accuracy, high spatial resoltuion (no resampling required), and low equip- 
ment cost (only one pixel at a time would be processed). 
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