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the process. The Green Kodak #58 filter replaced 
the Red and the Blue Kodak #47B replaced the Green 
to complete the process. It was noted that exposure 
changes were necessary for the GREEN and BLUE filters 
to match the Dx obtained on the initial RED separate, 
however, processing remained constant. These three 
exposures created three spectral renditions — RED, 
GREEN, and BLUE — of the original CIR positive 
images onto a black and white negative. 
The processed negatives were "sleeved" into plastic 
envelopes for protection and temporary storage. 
Due to the limitations of instruments previously 
described, the CIR negative film was not used in the 
production of orthophotos. Instead, the film was used 
to generate black-and-white film positives using 
Kodak 4421 (aerographic duplicating film). Horizontal 
and vertical control was annotated on the film by the 
Mexican authorities and then transferred to the RED 
separate via the Kern PMG-2 point transfer device by 
USGS personnel. The positives were then oriented 
stereoscopically in the Zeiss C-8 stereoplanigraph 
plotter and systematically profiled. Simultaneously 
the stereomodel and orientation parameters were 
digitized and recorded on magnetic tape for use on the 
Digital Profile Recording Output System (DPROS).* Each 
quad-centered exposure was scanned in this manner. 
With exposure and processing parameters established, 
differential rectification began with the first 
quad-centered RED separate. The negative was aligned 
in the plate holder via fiducial marks, (fiducial 
alignment of the separates in the plate holder was 
critical since the orientation settings are identical 
for all three separates), mounted in the Geigas Zeiss 
Orthoprojector (GZ-l),** and positioned using the same 
values established during the orientation and profiling 
of the positives on the C-8. A minor correction 
compensated the offset of bore-sighted cameras. The 
profile tape was loaded onto the playback console 
(reader) with the film*** positioned in the GZ-l. 
* USGS fabricated in-house system using magnetic 
tapes and a digital recording/playback console 
in conjunction with photogrammetric instruments 
capable of x-step over y-profiling modes. 
** Orthoprojector driven by a magnetic tape console 
reader (USGS DPROS) whereby digital information controls 
the profile scanning movements of the instruments 
providing a differentially rectified film copy to 
desired scale. 
*** Kodak S0-240 used to reproduce orthophotos via 
the GZ-l projector. This film was chosen for its depend 
able densimetric stability.