GVII-6 
PHOTOGRAMMETRIC ENGINEERING 
3. Stereo-projection Instruments: The use of the stereo-projection principle 
in photographic interpretation work received new impetus during the reporting 
period. In France and the United States, projection equipment in use permits the 
simultaneous viewing by several interpreters, through polaroid or analgyph 
glasses, of aerial photography projected stereoscopically. The French equip 
ment uses a metallic screen, has two projectors which project aerial photographic 
positive transparencies, and a third small projector which throws a movable 
light spot on the screen for pointing out image details. The U. S. equipment, 
shown in Figure 1 is a “rear-view” type projector in which the image is thrown 
on the opposite side of the ground glass screen from the viewer. Its optical 
system permits the stereoscopic projection of images from two adjacent frames 
of an aerial film transparency, without the frames being cut apart. Equipment 
of this type will permit rapid assessment of the results of a photographic mission 
to be made without the time-consuming handling of individual prints. As 
Fig. 2. The Micro-linear scale. A compact instrument developed to measure distances up to 
6 inches on photographs. The screw is calibrated to read linear motion of the magnifier directly to 
QjQfiD 1 feet. Wooden scale is shown for size comparison. 
earlier stated in this report, use is also being made of photogrammetric stereo 
projection plotters in photographic interpretation work. 
4. The Magnifying Measuring Scale: For some years the photographic in 
terpreter has been using scales mounted in monocular magnifiers for measuring 
small objects imaged on photographs. For measuring distances of several inches 
on photographs, he usually used a wooden, plastic or metal scale, viewing the 
graduations through a separately held magnifier, when necessary. During the 
reporting period, a combination scale and magnifier made its appearance. This 
device, shown in Figure 2, is a compact portable instrument, permitting fairly 
rapid measurement with very good accuracies. 
5. Stereo-elevation Meter: A simple instrument based on the parallax ladder 
principle, the stereo-elevation meter is designed to permit determination of 
elevation differences from vertical stereo pairs without resorting to mathemat 
ical computations. The device is pre-set by the operator for the particular 
photographs on which it is to be used; after this has been done, elevations are 
read directly. 
6. Stereo-slope Devices: A number of devices for measuring slopes from 
vertical photography came into use during the reporting period. One here 
described is the Super-Duper-Dipper, developed by Robert J. Hackman of the 
U. S. Geological Survey, and used in that agency. The device has two small