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into the computer. The latter include a basic program tape and an 
initial-conditions tape, as in the AP-2 system. An additional separate 
program tape is also entered, depending on whether the operational 
mode is to be profiling or contouring. 
Interior Orientation 
Approximate positions of the fiducial marks on each photograph 
are entered into the computer by means of the initial-conditions 
program tape. When the interior orientation procedure is initiated by 
the operator, the computer drives the photocarriages to one of these 
positions and begins a simple search routine to locate a reference 
portion of the fiducial mark, using information supplied by the corre 
lator. The correlator outputs are then used by the computer to control 
positioning of the photograph until the fiducial mark is correctly aligned 
with the optics. This location is stored, and the photograph is indexed 
to the next fiducial mark. After all fiducial locations have been de 
termined, the photograph center is computed automatically, adjusted 
for any known camera calibration errors, and then stored as the location 
of the principal point. 
Relative Orientation 
Relative orientation is based on measurements of Y parallax made 
automatically at from 5 to 12 points in the model. At each such point, 
the correlator outputs are used by the computer to eliminate both X and 
Y parallax. The X parallax is eliminated by controlling the effective 
elevation of the point being observed in the model in order to permit a 
more accurate measurement of Y parallax. The Y parallax is eliminated 
by inserting a differential displacement in the Y positions of the two 
photographs. During this process, the amount of motion required is 
recorded as the Y parallax. After these measurements have been made 
at the desired number of points, the computer calculates and stores in 
appropriate locations the necessary changes in the values of the orien 
tation elements. 
Absolute Orientation 
Absolute orientation is based on the known model coordinates of 
three or more control points. It is performed in the same manner as 
in the basic AP-2 system, except that model elevation is now automati 
cally controlled to eliminate X parallax. Any residual Y parallax is