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9 
Photogrammetry on a part of a forced conduit pipe. 
THE PROBLEM : Take stereoscopic photographs of a cylindrical surface of a tube, of the inner 
and the outer wall, so as to deduct the thickness on each point of the section by the photogram- 
metric method. 
The problem consists in superposing upon each of both stereoscopic photographs a grid, so 
that on each point of the same co-ordinates the sum of the corresponding marks gives the thick- 
ness of the tube, not in the usual vertical photogrammetric way, but according to the curve of 
the two faces. 
OPERATION TECHNIQUE : As to the joining of the picture pairs in relation to the reference 
plane, the latter had to be materialized so that in each negative this plane should be visible, in 
order to permit the absolute orientation of the pair. 
For that purpose, two metric steel tapes, visible on the upper and bottom border of each 
negative, have been chosen and photographed, so as to materialize a reference plane. 
This principle yet wants some precautions to be taken during the photographic operations. 
1°) The two tapes have to be put under a known tension (10kg on each tape). 
2°) The height of the attaching and fiducial marks must be known so as to allow to deduce from 
it the interval points, because of the inevitable curve caused by the weight of the tapes 
3°) A second control has been made by levelling all the points necessary for plotting, by a first- 
rate levelling from two symmetric stations in conditions similar to the position of the tapes 
during the photographic operations. 
4°) For every reference plane (inside and outside), a metallic frame fixed on the tube has per- 
mitted to establish the parallelism and the continuous symmetry of the two curved reference 
surfaces. 
5°) Afetr the plotting of the stereo-pairs, the contourlines drawn in a plane which is uniform 
and perpendicular to the central radius of the section must be represented in a projection 
system allowing the direct reading of the thickness of the wall. 
For that purpose the X, Y system is modified ints definite sections, of which the values 
change according to the angle formed by the position upon the circumference and its centre. 
After giving the basic elements of this photogrammetric application, I beg the persons inte- 
rested in this matter to refer to the French text for any further information.