Full text: Transactions of the Symposium on Photo Interpretation

and annotating geologic data. The larger scale maps are also reduced to 1 : 
250,000 scale to form part of a general-purpose small-scale series of maps. 
Photo interpretation is extensively used in general-purpose mapping-in com 
piling the planimetric map, in sketching the main topographic features with 
contour lines, and in shaded-relief representation. 
The use of photo interpretation for planimetric mapping may be described 
in four main steps: 1. A systematic stereoscopic examination and identification 
of the terrain features, and their delineation upon the photographs; 2. detail 
points are selected under the stereoscope for controlling the transfer of terrain 
details from photograph to map; 3. the detail points are located by means of 
plotting equipment; and 4. transfer of terrain features from the vertical and 
oblique photographs to the controlled map by means of sketchmasters. 
In topographic mapping, additional stereoscopic studies are used for selecting 
critical elevations on the oblique photographs and for contouring. The photo 
alidade, a monoscopic plotting instrument, is used to obtain elevations for con 
touring. Contours are sketched by hand, as in plane table mapping, after yet 
another stereoscopic examination of selected vertical and oblique photographs. 
Photographs from more than one flight are often used to control the sketching. 
Equipment used in photo interpretation consists mainly of various types of 
stereoscopes, sketchmasters, projection equipment, and plotting equipment. 
Shaded-relief representation 
Since many ice and snow features tend to blend into a common ice back 
ground, shadows sometimes become the most significant characteristic of the 
features. Some form of cartographic shadowing or shaded relief becomes a 
necessity in portraying ice and snow features under these conditions. An ac 
curate photo interpretation of such features and their shadows is of utmost im 
portance in the correct portrayal of shaded relief. 
The shaded-relief specialist makes a complete stereoscopic examination of 
all vertical, oblique, or ground photography of the pertinent area. Using the 
available elevation data, he compensates for inherent displacement or distortion 
of the photographic detail, and visually adds additional details which may be 
needed to bring out the character or pattern. The length of the shading is 
made approximately proportional to the height of the feature above the ground. 
The shading is drawn at a 45° angle, with the apparent light source in the 
Undulations in the ice or snow, glacier flow lines and stress lines, disturbed 
ice, hinge-lines in the ice, and crevasses, both singly and in systems, are some of 
the ice features which can be more clearly expressed bv shaded relief than by 
conventional map symbols. 
The final representation is made by a combination of airbrush, pen and ink, 
pencil, and white paint. Subsequent cartographic scribing and map reproduc 
tion is coordinated with the shading to preserve the symbol and color value of 
the shading. All United States Geological Survey general-purpose Antarctic 
charts at 1 : 250,000 scale and smaller now carry shaded-relief representation.

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