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permit material variation in the scale of the image it produces. The scale of the 
model is usually the maximum that other considerations, in the design of the 
instrument, will permit. 
Apparently neither type of plotting equipment has reached its perform- 
ance peak. Many new, and according to the advertisements, improved models 
of the optical train plotters have appeared on the market within the last year, 
and it is quite likely that the near future will witness similar changes and im- 
provements in the projection plotter field. 
The first generally accepted projection plotting equipment using miniature 
diapositives certainly offered no challenge to instruments of the other type on 
the basis of attainable accuracy alone; but other factors including cost and pos- 
sibility of mass production so outweighed any such deficiency, that these 
instruments became firmly established in the field of photogrammetry though- 
out the world. The many things that were learned through use offered a 
splendid base for the subsequent development of the Kelsh Plotter. 
The Kelsh Plotter uses full scale diapositives that may be printed by con- 
tact, thus eliminating the need for an expensive printer. The lighting system is 
designed to concentrate all its light on the small area of the platen of the 
tracing table, the light following the tracing table automatically as it is moved. 
This development made it possible to reduce both the aperture of the projec- 
tion lens and the size of the illuminant. The result is plenty of light, consider- 
able depth of focus (one of the weak points in projection apparatae), and no 
heat problem. 
The light guiding arms furnish a means of working an automatic cor- 
rector for lens distortion, either of the taking lens or the projection lens. By 
suitable mechnical linkage the Projection lens moves along its vertical axis, thus 
increasing or decreasing the distance between the diapositive and the lens. 
While such simple correction method is not 10076 theoretically correct, the 
amount of error, within the range of the instrument, does not reach the point 
of limit of visual perception, and in practice the device has been very satis- 
factory indeed. 
Factors for earth curvature may also be included in the correction device. 
This feature is of increasing importance as our data is secured from higher and 
higher altitude. 
The instrument, without alteration, may be used with convergent photo- 
graphy up to 20-25 degrees, and a model for use with trimetrogon wing photo- 
graphs is in development. 
Considerable increase in flexibility was secured by adding à pantograph. 
As designed, this permits only reduction from the model scale. 
Means for recovering the tip and tilt of either of the pair of diapositives, 
by use of level bubbles, make it possible to use the Plotter for extension of 
horizontal and vertical control. 
Now the features that have been mentioned definitely represent improve- 
ment in this type of plotter, but just how much improvement it is surprisingly 
difficult to assess. 
The instrument has been accepted by practically all the large mapping 
organizations in the United States of America, both commercial and govern- 
ment, including the military. But 
the results of military tests are necessarily 
restricted and those of competing c 
ommercial concerns are, as might be expect- 
   
    
  
   
   
   
   
  
    
   
   
  
   
    
   
    
    
  
   
  
   
   
   
  
    
  
  
    
   
    
   
     
   
    
    
   
   
    
    
  
    
   
   
    
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