REPORT OF COMMISSION VII 
G VI1-7 
identical targets which are fused stereoscopically into a single target. The fused 
target is raised or lowered with respect to the stereoscopic model by varying the 
horizontal separation between targets. However, the angle of slope is deter 
mined by the actual physical tilting of the targets in space. The device is used 
in conjunction with the Supplementary Slope Model, a set of line drawings 
which provides instant correction for the vertical exaggeration induced in the 
original reading by factors of focal length, photo base, and image separation. 
A second ingenious device for rapid measurement of slopes is also in use in 
the U. S. Geological Survey. Developed by William R. Hemphill, it consists of 
a tilting platen for use in Kelsh or Multiplex-type stereo projection equipment. 
In contrast to the usual horizontal platen, the tilting platen has a hinged joint 
above its base, and can be tilted to any angle up to 42 degrees. The surface of 
the platen is patterned with dots or small crosses, to enable the operator to 
relate it more clearly to slopes in the steroscopic model. The angle of inclination 
is measured by means of a clinometer. 
7. Plastic Scales: Many photographic interpreters are using “packs” of 
graphic measuring scales, printed on plastic transparent sheets, and pre-com- 
puted for various scales of photography. These are widely used in Europe and 
the Americas in such work as timber surveying, where precise measurement of 
distances is not required, and where speed is essential. 
8. Sampling Guides: Plastic guides for various types for statistical sampling 
have been devised and used by interpreters in various fields. These vary widely, 
depending on the subject of interpretation, and the sampling procedure to be 
allowed. Most are for use over single photographs; however, a stereoscopic 
sampling device, consisting of a series of triangles etched on two glass plates for 
estimating forest crown diameters, has been developed in Canada by S. T. B. 
Losee. The stereoscopic facility in this case permits the sampling templet to be 
“floated” at tree-top height for more accurate reading. 
9. Instruments for Oblique Analysis: While the analytical, graphical or 
mechanical rectification of images from oblique photography is, strictly speak 
ing, a photogrammetric matter, it is also of considerable interest to the photo 
graphic interpreter who must study and measure oblique photography in the 
course of his work. This reporting period saw much emphasis being placed on 
simple and rapid methods of handling oblique mensuration problems, specifi 
cally for photographic interpretation work. The results of these investigations 
included the development of at least one mechanical plotter, and number of 
graphical techniques involving such pieces of equipment as sets of plastic 
overlays, and nomograph scales. Development in this field is still going on. 
Personnel and Training 
SELECTION OF PHOTOGRAPHIC INTERPRETERS 
The problem of selecting photo interpreters has recently become a topic 
of serious thought. Colwell (1954) discusses this under two main factors—visual 
acuity and mental acuity. Visual acuity he defines as the ability of a photo 
interpreter to perceive a photo image by the characteristics of tone or color, 
image sharpness, and stereoscopic parallax. He point out that there is a “thresh 
old range” of contrasts, in which a rather slight increase in relative contrast 
between an object and its background greatly increases the probability of 
detecting that object. Furthermore, the human eye seems to be more sensitive 
to the light end of the gray scale than to the dark end. Also the human eye is 
found to be more sensitive to the green and yellow light than to the blue and red 
parts of the visible spectrum.