-3- 
presents a short description of the modifications to the automatic parallax 
removal in order to improve correlation and provide the slope compensation 
an d rectification required for the orthophoto printer. 
Figure 4 shows a block diagram of the scan and parallax discrimination 
circuit, the output of the X and Y random noise generators {I and III) are 
combined together with the output of a sine wave generator (II) to obtain the 
X and Y deflection signals. These signals are amplitude controlled in the 
pattern size circuit (IV and V) by the correlation signal from the registra 
tion discriminator. The outputs of IV and V are then fed to the scan correc 
tion amplifiers VI and VII which modify the X and Y deflection signals for each 
CRT using the terrain slope and photo orientation signals from the correction 
computer (C). This results in the scan pattern on each tube having the same 
relation to an unmodified scan pattern as the actual diapositive image on each 
side has to a truly orthogonal representation of the terrain. 
The output from the scan correction amplifiers is now fed to power ampli 
fiers (VIII to XI) which provide the current to the X and Y deflection yokes on 
each cathode ray tube. 
The scanning pattern on both CRT's, consisting of a random noise scan 
superimposed upon a circular scan, are now focused upon the diapositive. 
They are intensity modulated by the density variations there and projected on 
the photo multipliers and , where they are converted into amplitude modu 
lated electrical signals, which are then fed to the registration discriminator D. 
Registration Discriminator D 
The registration discriminator D has the following outputs: 
(a) The Correlation Signal: 
When both CRT's scan areas with identical information, (i.e., at any one 
time the projection of the CRT beams on both diapositives passes over identical 
light-dark boundaries) the signals from P^ and P^ will be identical and there will 
be a maximum correlation signal. This signal is used to reduce the scan ampli 
tudes or pattern size, (in IV and V) so as to limit the area scanned and obtain the 
highest possible measuring resolution. 
(b) Parallax Signal: 
If plate I is moved slightly in the X direction, with respect to plate II, one 
of the CRT beam projections will pass over identical light-dark boundaries later 
than the other. This will cause a time delay or phase shift to appear between the 
output of P-ji and P^. This phase shift is detected in D and causes an X parallax 
output signal. Similarly a Y parallax signal is obtained. The Y parallax signal