142 
THE CANADIAN SURVEYOR 
The Stereo Line Plotter 
The instrument is designed to give a linear function in dZ, that is, we ignore the Ap in 
the denominator of equation (1). Using this equation and substituting in it the camera focal 
length f for Z and the photo base b' for the airbase b, equation (2) in photo coordinates is 
a 2 Z xyZ xZ yZ ( // \ 
dZ — A</> ~| Aui -| (Abz — 2b'd(j)') -\ ( An -\ d<x>" ) (3) 
b'f b’j b'f b'\ f ) 
Fig. 1. The well-known representation of the deformation in height expressed by equation 3. 
The problem then was to build an instrument that would compensate for the model de 
formation in height as shown in equation 3, that would be inexpensive to construct, easy to adjust 
and transport, and would enable a topographer to contour quickly with minimum effort and 
computation.