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Reprinted from 
PHOTOGRAMMETRIC ENGINEERING 
April 
1956 
Experiences with Convergent 
Photography" 
WILLIAM C. CUDE, Chief, Topographic Engineering Dept., 
Engineer Research & Development Laboratories, 
Fort Belvoir, Va. 
HE study and application of twin camera convergent aerial photography for 
T purposes is not new. It has been used by several European coun- 
tries and would have realized a much more universal application except for 
the introduction of the wide-angle mapping camera lens. Most photogramme- 
trists, who would otherwise have resorted to convergent techniques, were at- 
tracted by the advantages of the wide-angle lens for a solution to their problems 
and, except for a very few, interest in convergent photography was lost. 
In the U. S. where a great part of the country is yet to be mapped satisfac- 
torily at scales of 1:25,000, or even smaller, an ever present pressure exists to 
find methods of producing topographic maps with the highest possible accuracy, 
but with maximum speed and economy. 
Recently, the question occurred to photogrammetrists in the U. S. that if 
convergent normal-angle photography improved performance and economy of 
nomal-angle techniques, why shouldn't convergent wide-angle photography 
improve the performance and economy of wide-angle techniques? Superficial 
inspection of usage of convergent wide-angle photography was very interesting 
and promising. The greater base height ratio promised higher vertical accu- 
races; the coverage of twin convergent camera installation indicated economy 
of photography, stereomodels, and ground control. There were difficulties 
anticipated too. Convergent photography in terrain of considerable relief 
could cause holidays; two cameras are required which of course means more 
opportunity for failure: contact prints are not usable for mosaic work or control 
identification—questions arose as to what equipment could be utilized, how 
could aerial triangulation be effectively accomplished. However, the advantages 
appeared to outweigh the disadvantages and studies were initiated by the U. S. 
Geological Survey and the U. S. Army Corps of Engineers to evaluate the full 
potential of convergent wide-angle mapping photography. It was agreed by both 
organizations very early in their investigations that a convergence angle of 40 
degrees (each camera 20 degrees from the vertical) appeared to be the optimum 
arangment for wide-angle lenses. It was also determined that the stereo- 
plotting equipment most commonly used in the U. S., the direct projection, 
dichromatic anaglyphic instruments, namely the Multiplex, the Kelsh Plotters, 
and the ER-55 or Balplex plotter were exceptionally well adapted to convergent 
Photography with a minimum of modification. This fact no doubt played an 
portant part in the acceptance in the U. S. of the idea of convergent photog- 
raphy. 
The first investigations of the application of 20-degree convergent wide- 
angle photography made at the Engineer Research and Development Labora- 
a * This paper is a contribution to Commission III of the International Society of Photogram- 
ry, 
U.S.A.-1 
  
Appendix to the INT. ARCHIVES OF 
| PHOTOGRAMMETRY, Vol. XIE4, 1956 
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