9 
e. The operator then moves the photocarriage over against mechanical 
stops on the left-hand side of the Steretope base plate and pushes a button 
to signal the calculator to read the X, Y measurements from the Datagrid. 
This established in the calculator the origin or point of reference for the 
system. The Identification number for the stereo pair is now entered in the 
calculator keyboard by the operator. The calculator automatically locates 
the data stored on the tape cassette for that stereo pair and loads the data 
into the calculator memory. 
f. The operator views the left photo monoscopi cally with the Stereotope, 
centers over an index point, and signals the calculator to read the X, Y 
measurements from the Datagrid. This is repeated for each of four index points 
on the left photo plus four index points on the right photo. The calculator 
computes a transformation that converts Datagrid measurements into photo 
coordinates. 
g. As previously indicated, the system is designed to be self-checking. 
Viewing both photos through the Stereotope, the operator centers over a check 
point while adjusting the X-parallax and Y-parallax controls to obtain the 
necessary stereoscopic relationship between the photos and signals the calculator 
to read the X, Y measurements from the Datagrid plus the Z (elevation) 
measurement from the shaft angle encoder. The calculator then uses the data 
from the tape cassette and the X, Y, Z measurements to solve for the position 
and elevation of the checkpoint. The calculator automatically compares the 
computed values with the known values for the checkpoint. If these values