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mute enses 
CLASSIFICATION OF INSTRUMENTS, SCHERMERHORN 
sion, but mainly in the construction, which results in the necessity to apply special meth- 
ods for aerial triangulation. The consequences of the design are a few complications in 
the execution of the triangulation but the precision of the results does not suffer from 
We believe that the name “precision plotters’ 
the usual indication 
this proposal sounds quite good for this group of instruments, we believe nevertheless 
that the precision is not a very suitable criterion for classification. To prove this let us 
consider the instruments with direct optical projection of the Nistri-Kelsh type. Is its 
precision the striking feature which invites to distinguish this from other instruments? 
Certainly not, since we experienced that with well-adjusted projectors and a flat table a 
standard deviation in the measurement of height in precise grid models of less than 
0.19/,, of the flying height can be obtained. This would bring these instruments into the 
class of precision plotters. With actual photography, however, the difference in precision 
between this type and what we called above precision plotters is larger. An American 
C-factor of 1000 for the Kelsh type is normal against 1500—2000 for precision plotters. 
I believe that this is due to the difference in the system of observation: anaglyph pro- 
cedure with naked eye observation against stereoscopic observation in transparent light 
with 6-9 times optical enlargement. 
The consequence of these considerations could be to abandon the precision of the in- 
struments as obtained in grid models as a criterion for classification and to use only that, 
obtained with the height measurement in actual photographs. This would bring us very 
close to the thought of accepting the American C-factor as criterion for a classification 
of instruments as seems to be usual in USA practice. The smallest possible contourline 
interval however, depends not only on the qualities of the plotting instrument, but also on 
the entire chain of steps in the mapping procedure. In particular the quality of the 
images offered to the machine are of great importance. Whether we use old-type topogon 
or metrogon photography on a 9” X 9” film or photographs taken with the best modern 
lens on ultra flat glass plates can change the result by a factor 2. To demonstrate how 
impossible it is to operate along this line we have only to study the results published. 
I remember one publication in Photogrammetria which gives for plotting in the old 
Wild Autograph A 6 a C-factor = 1800. Is now the A 6 a first-order instrument? Not at 
all. A recent American publication on the average of a number of large scale projects 
highway engineering in California with Kelsh plotters results in C = 720. Is the 
Kelsh plotter now a third-order instrument? Of course not, only the C-factor as deter- 
mined from actual mapping is useless for classification. We could prove this also by using 
examples from multiplex plotting where a C-factor of 500-600 was found. Nevertheless, 
we cannot classify the Multiplex as a third-order instrument since this expression is 
used for instruments with an approximate solution. 
All these examples show that the usual distinction on the basis of precision between 
various types of plotters cannot be accepted, neither for grid models nor for actual pho- 
tography. The difficulty increases even more if we look at the increasing number of plot- 
ting instruments designed for special purposes in particular for topographic mapping. 
London we will 
Wild Aviographs B8 and B9, Kern PG2 and a Zeiss instrument. Looking at these in- 
struments in relation to their proper function we could call these also “precision plot- 
ters”, because they will without any doubt attain a higher precision in planimetry than 
required and for height come also close to the 0.19/,, of the flying height. 
With the further specialization of the instruments we must evaluate the instru- 
ments in relation to the task for which they were designed. To prove this I could have 
also used other instruments like the Galileo Santoni Stereosimplex II, the SOM Poivil- 
liers Type D and the SOM-Baboz plotter. 
   
   
  
  
     
   
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
    
   
  
  
  
  
   
  
  
  
  
  
  
  
  
  
   
   
   
  
   
  
  
  
    
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expresses their qualities better than 
second-order plotters”, which is misleading in this respect. Although 
«c 
see instruments with a correct solution for topographic mapping: