2 
surface. The projector uses a wide angle lens, typically 
of six inch focal length. The elevation corrections 
are introduced by moving the entire table up and 
down, while the scanning is accomplished by a slit 
traveling back and forth. The slit intercepts a 
small element of the large cone of illumination, 
while the balance of the area is masked. 
The Off-Line Printer is essentially an analytical 
printer desianed in the concept of the analytical 
plotter. The ootical system is fixed and has a narrow 
field of view. The bundle of light rays does not 
suffer from high refractive ancles. The illumination 
is a thin slit of light created by projection of 
the actual filament of the illuminating lamp upon 
the input photo (Figure 1). Line elements of the 
imagery are transformed by a zoom lens and a dove 
prism and reimaged upon the output film. This trans 
formation also involves the computation of the proper 
photo coordinates on the input photo corresponding 
to the required model coordinates on the output film. 
In the scanning/printing process the control computer 
generates the appropriate signals to control the 
X and Y input photo position, zoom setting, filament 
and dove rotation, and X and Y positions of the output 
drum, referred to as model coordinates. Thus the 
model coordinates are the orthophoto coordinates. 
MODES OF OPERATION 
There are two basic modes of operation - ortho 
rectification and plane rectification. The orthorecti 
fication process, a differential rectification, is 
the most complex of the two and results in properly 
relocating image points from their displaced positions 
in the original photograph - based upon photo geometry 
and relief. The result is essentially a photomap, 
if the appropriate reference grids are annotated 
on the record. (Figures 7, 8, 10.) 
The rectification process is a special case 
of orthorectification and is accomplished in the 
same manner on the printer, with the elevation being 
held constant. (Figures 12, 14) 
A second special case of image transformation 
has been accomplished with the Off-Line Printer by 
an additional software package. This product is 
a stereomate (Figures 9, 11) conceived and developed