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This has happened recently for one of our best lenses. 
For many years we have battled against poor centering of lenses, against 
unsymetrical distortion and tangential distortion of our lenses. (Sometimes as 
much as several minutes). A research engineer employed by the factory dis- 
covered and published an improved test. At first we thought we could obtain 
the desired tolerances only by selecting a few of the bes: from a large number 
of lenses. Later the manufacturer found the means of keeping the tolerances 
for as many lenses as desired. There resulted a real and important technical 
improvement. 
We progres by complete and frank discussion of all the difficulties we 
encounter in obtaining the precision we need and by adopting standardized 
tests to insure the best operating conditions. Each time we adopt better prac- 
tical tests and more rigid tolerances we establish a base from which we can 
attain still higher precision". 
From 5.35 to 5.44 Mr. Schermerhorn (Netherlands) responded to the ideas 
of Mr. Reading. His words can be summarized as follows: *I would say that 
I am not in disagreement with Mr. Reading. It is useful to have open discus- 
sion in technical literature on the best methods of quality control in photo- 
grammetry, and, if I have criticized the project of Mr. Pennington, it is only 
because his project simplifies the problem in a way that is not most compatible 
with the task of research. It is necessary that we know our apparatus as well as 
possible and the adoption of test plates is an excellent contribution to the 
problem. The Review “Photogrammetria” is open to anyone who wishes to 
contribute to such solutions. As M. Cruset has said, we do not know a great 
deal about the possibilities, the production of the machines. That is true, it is 
necessary to improve the situation in this direction. But if we would compare 
not only one step of the survey but the complete series of operation, we 
encounter this difficulty. For example at Delft, we do not have in regard to 
French apparatus the same experience that you have at the Institut Geographi- 
que National; that is to say definite results for the comparison of the methods, 
the instruments; there is no practical solution but this. Many publications treat, 
for example, of aerial triangulation but do not permit of comparing the value 
of the different methods. We have tried in Europe to establish under the 
auspices of the OEEC a working center which will permit progress to be made 
on this question. M. Cruset knows well the difficulties to be solved; valuable con- 
clusions will not be obtained except by a direct international collaboration 
between the institutes and organizations which have extensive experience witb 
their methods and apparatus. 
There is another difficulty: numerous publications refer to work executed 
on old instruments before the last war. These have no value if one would judge 
the possibilities of current apparatus. I think it is necessary to do something to 
progress, but my feeling is that, to arrive at the solution of photogrammetric 
problems, the results obtained until now are too slow and this for lack of 
effectual international collaboration. A meeting like this Congress certainly 
permits exchange of ideas and useful discussions, but what it does not bring 
about is a complete comparison of the work done by different institutes or 
organizations with the methods and equipment in which they are most skilled. 
Each congress — and this is now my sixth — has discussed these questions, but 
it Is necessary to do something positive otherwise in 4 years at the next congress, 
  
  
  
  
  
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