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US A2 PHOTOGRAMMETRIC ENGINEERING 
tories were with the Multiplex equipment. Simple mechanical adapters were 
designed to permit tipping of the projectors the required angle; no other modif. 
cation of the equipment was required. It was necessary to modify the relative 
orientation procedure somewhat, but this caused no confusion to the Opera- 
tors. One phase of the investigation was a comparative stereoplotting test Where 
the same area was photographed with conventional wide-angle vertical tech- 
niques and simultaneously with convergent methods. This test revealed thai 
with the convergent photography, the contouring accuracy of the Multiplex 
equipment was increased from 1.5 to 2 times that of vertical photography, No 
increase in the planimetric accuracy was observed. The remarkable increase i 
performance of the Multiplex brought the next question to our consideration, 
How much could the stereoplotting potential of the Kelsh Plotter be Improved 
if it were adapted to convergent photography? Here again it was fortunate that 
the Kelsh Plotter required very little modification to make it suitable for use 
with 20-degree convergent photography. As in the Multiplex test, there was 
prepared two maps of the same area with simultaneously taken photography, 
using the same type cameras, except of course one coverage was vertical and 
the other convergent. Again discovered was a remarkable increase in contour- 
ing and heightening accuracy, 1.5 to 2 times that with vertical stereo models, and 
no change in planimetric accuracy. 
It was observed in the stereomodels formed with convergent photography 
that a considerable fall-off in resolution existed. This condition was more 
noticeable in the Kelsh Plotter than the Multiplex. This is understandable 
because of the greater magnification in the Kelsh Plotter system and also the 
depth of focus of the Multiplex projector lens is extremely great. This fall-off 
in resolution is caused by tipping the plane of best focus of the projector twenty 
degrees to the datum. Special wedge shaped plate holders were designed to 
improve the resolution by satisfying the Scheimpflug condition. This was done 
by tilting the plane of the diapositive to intersect with the common intersec- 
tion of the datum plane and the lens plane. Care was taken to see that the 
principal point of the diapositive coincided with the foot of the perpendicular 
from the projection lens. Use of these special plate holders increased the resolu- 
tion of the projected image considerably. However, and surprisingly, the 
contouring and heightening accuracy of these more highly resolved stereomodels 
did not differ significantly. Here was encountered a problem which has been met 
before by many photogrammetrists but to our knowledge has not been answered 
in a definitive quantitative manner, to wit; what is the correlation between 
resolution and stereo-acuity? This problem is being investigated at the Labora- 
tories at present. 
It was recognized early in the investigation that for convergent photography 
to be of real practical use in solving the peculiar mapping problems, that is, 
covering great areas with maximum accuracy and minimum time and expense, 
it would be almost mandatory that such photography lend itself to accurate 
aero-triangulation. Accordingly another phase of the investigation concentrated 
upon methods and instrumentation required to successfully bridge control, 
both vertical and horizontal, with accuracies commensurate with the stereo- 
plotting capabilities. This was not so simple an investigation as stereoplotting, 
but then any form of aero-triangulation is hardly as simple a task as the plotting 
of an individual stereomodel. The initial studies of possible methods of aero 
triangulation concerned principally establishment of vertical control. It was 
here that most difficulty was anticipated. Conventional methods for bridging 
horizontal control could always be resorted to if necessary. Slotted templet and 
  
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