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On the base of SD-3 and SPR-3, more perfect models 
of universal plotting instruments are being elaborated. 
Machines for differential rectification: 
  
Differential photorectifier DFT.- Is intended for 
automatic rectification of photographs of highlands 
under utilization of previously obtained information. 
DFT permits to eliminate the errors on the boundaries 
  
of strips of the  orthophotograph as are caused by 
height differences of points within elementary areas; 
this enabled to triple the width of Strips of the ortho- 
photograph. The principle of affine rectification made 
use of in this instrument makes it possible to plot 
photographs of any focal length. The DFT consists of an 
orthoprojector and an automatized control device. The 
orthoprojector is a single-picture optical- mechanical 
instrument. Introducing of a photograph in the ortho- 
projector is fulfilled according to its orientation 
elements,as are read on the scales of the analog instru- 
ment when plotting a stereopair to which the given 
photograph belongs. The orthoprojector is working under 
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ch makes use of in- 
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guidance of an automatic control whi 
formation registered on a perforated tape. This infor- 
mation is obtained with the help of an electronic com- 
puter from a digital model being plotted on the analog 
machine simultaneously with contour drawing. 
Orthophotoprojector OFPD.- Is based on the principle of 
the SD-5 stereograph. The scale of the orthophotograph 
obtained with this instrument is equal to that of the 
model. Thanks to its light-tight design, the instrument 
may be used in normal light conditions. The magazine may 
be loaded either with plates or film. When plotting 
photographs, profiling is carried out by hand, i.e. 
the stereoscopic image of the mark is made to coincide 
with the stereomodel by the aid of & wheel X, connected 
in this case with a drive screw.