2 
nical projection of the lines of sight in aerial photogrammetry appeared at the time to 
be devoted to failure. 
The solution to the problem was afforded by introducing in the plotter the means for 
correcting the effects of the distortion from the camera lens. 
Already in my first plotter utilizing a mechanical projection (the *Autoriduttore") 
constructed in Turin in 1920, I had the guide-rods shaped in a particular way that pro- 
vided compensation for lens distortion, although limited to one type of objective. 
In 1925 however, I devised my mechanical method of distortion correction which I 
used on my first stereoscopic plotter (the ‘“Stereocartograph Mod. I, constructed at the 
Italian Military Geographic Institute, where it was successfully in operation for many 
years). 
Opportunely modified, the dis- 
tortion cam correcting device was 
duly protected by an international 
patent in 1928. 
It can be stated, I think, that 
my distortion corrector led the way 
to the development of those univer- 
sal spacial plotters that utilize a 
totally mechanical projection. 
Operation of the distortion cor- 
rector is known. The device results 
(see fig. 1) of a special cam of re- 
  
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RN i N Tí) 
  
  
volution (c) provided on ball-joint "E E . 
(g) cardanically connecting guide- 7 / X x 
rod (d) to either the photograph- 
holder, in plotters that utilize a fixed Fig.1 
optical system and movable photo- 
graphs (such as the Stereocartograph Mods. II and IV and the Stereosimplex Mods. II 
and III) or the last element in the optical system of observation, in plotters utilizing 
fixed photographs and movable optical system (such as the Stereocartographs Mods. I 
and III). 
A special cylinder housing is provided for ball-joint (g) which has a vertical motion 
over a segment that runs parallel to the camera axis. The cam surface is kept in contact 
with a small fixed ball (s) by means of a spring system (m). Therefore, as the angular 
direction of guide-rod (d) is changed with respect to the camera axis, the area of contact 
of cam (c) with fixed ball (s) is also changed, which generates a displacement of ball (g) 
center along the cylinder axis, and therefore, a mechanical variation of the principal 
distance, as a function of the cam generating curve. 
The cam is therefore so constructed as to reproduce, according to the different angles 
of field, the variations of principal distance that correspond to the distortion curve of the 
camera lens. 
The cams are manufactured using a special small lathe (see fig. 2) whose cutting 
tool (u) has a constant profile equal to the section of the fixed small ball, and is allowed 
radial micrometric displacements controlled by a graduated drum. Circular graduation 
(h) is used to control the angle according to which the cam radius shall be changed, so as 
to correspond to the variation of principal distance to be provided. The cams manufactur- 
ed in indeformable steel, are subsequently hardened and then checked by means of a 
centesimal micrometer, replacing for the purpose the cutting tool of the lathe. 
In the latest models, à small emery-grinding wheel replaces the cutting tool so that 
grinding of the hardened cam can be provided. Thus the principal distance variation can 
be actuated to the accuracy of — 3 micron. As the focal length variation affects distor- 
tion in function of the tangent of the angle formed time from time by the guide-rod with