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A Historical Note 
  
Almost precisely forty-five years ago - on 8 July, 
1927 - the French engineer R. Ferber made a patent appli- 
cation in Germany for an instrument capable of producing 
"un plan photographié" or for "obtention photographique 
de la projection orthogonal d'un objet". [1] 
In principle, the orthophoto technique presently 
used is only an adaptation of the solution suggested and 
demonstrated by Ferber. He used a stereo-plotter of his 
design, built by the French company Gallus. In the space 
of the absolutely oriented model, Ferber arranged a slit 
through which an elementary portion of the photographic 
image of the terrain could be recorded on a film as the 
slit was moved along parallel terrain profiles. The pro- 
jection distance was varied by changing the height of pro- 
jectors. Ferber, who recognized fully the effect of the 
size of the slit on the geometric correctness of the "plan 
photographié" provided the facility of rotating the slit, 
without changing its effective length, to make it parallel 
to the main course of contour lines in a given zone of 
the stereo model. It follows that. the direction of scan- 
ned profiles could also be varied accordingly for a given 
zone. In the enclosed Figure, the Ferber-Galus plotter 
(without attachment to produce the orthophotos) is de- 
picted. It must be noticed that this instrument was 
equipped with automatic focusing devices which offered a 
good definition of the projected images disregarding the 
continuously changing projection distance. Further de- 
tails of Ferber's solution and the quality of his differ- 
entially rectified photographs are most impressive and 
compare favourably with present day products. The dif- 
ficulty encountered by Ferber was of a somewhat different 
nature and it resulted from the contemporary limitations 
of some stereo plotters including the plotter of his own 
design. 
The photographs placed in the projectors of 
Ferber's plotter were projected in "white" light on the 
projection table. As a result, both images were observed 
by each eye of the operator, contrary to the requirements 
Of the stereo-perception. By varying the projection 
distance, the horizontal parallax between corresponding 
model points was eliminated. This is a relatively simple 
operation for well defined, discrete model points.  How- 
ever, for featureless portions of terrain, this is a 
cumbersome and unreliable procedure and not suitable for 
continuous profiling of the model with a simultaneous 
photographic exposure of the image projected through the 
moving slit. 
Ferber overcame this difficulty by first record- 
ing the profiles. In a separate operation, he produced 
differentially rectified photographs by projecting only