8 
errors in plotting. Perspective projection may further be 
impaired by the use of a sealing glass in the airplane bottom* 
Controlled experimentation has proved that familiar models 
of deformation affecting the taking cone of rays are insuffi 
cient to describe the actual conditions. This suggests that 
the individual physical properties of an air current disturb 
ed by the aircraft influence the photographic taking geometry 
in a significant way. Further impairments of the photograph 
may result from instabilities of the emulsion base, especially 
when film is used. 
"Quasi-distortion" in analogue plotters 
By far most of the analogue plotting machines reconstruct 
■fhe two taking cones of rays to generate an optical model. 
If this is effected by means of a projection lens, i.e. by 
optical projoption, it will be self-evident that this process 
involves deformations analogous to those occurring in taking 
the photograph. Even where this reconstruction is implemented, 
by mechanical projection, using either spatial guide rods 
or tv/o-dimensional systems of straightedges for two projection 
planes, there occur error patterns of a quite similar nature. 
Possible causes are deformations of space rods (frequently 
due to their own weight) and unavoidable residual errors 
in the space rod gimbal pivots (Fig. 3). 
The compensation of systematic model deformations in plotting 
Attempts at correcting systematic imaging errors by instrumen 
tal means have so far made use of two devices, in the first 
line. One is the optical\correction plate inserted into the 
projection beam of the plotting machine as an optically 
refractive element to compensate for any residual '’distortions 
and for effects of earth curvature and atmospheric refraction; 
the other is a calibrated focal plane grid (reseau) photo 
graphed along with the picture, the known reseau cross posi 
tions serving to improve the picture geometry.