Each photograph is centered on a quarter section of a standard USGS 7.5-minute topographic map. The photographs are mostly free of clouds, and their resolution is high. A deer hunter, for example, couldn't spot an individual animal in the photo, but surely could make out a grove of a few trees along a likely looking draw. Using the contours on a topographic map, the deer hunter could tell the elevation and slope of the spot with some precision. Our deer hunter can also use an instrument called a stereoscope to view two overlapping photographs at once, rendering an image that appears to be in three dimensions.
Inexpensive stereoscopes are readily available from scientific supply houses.
Instructions for their use can be found in any number of standard texts on cartography, terrain analysis, remote sensing, and photointerpretation. Because NAPP photographs are used for cartographic and analytic purposes, they are always taken with sufficient overlap to create stereo pairs. Any buyer of the photographs who wishes to see the three-dimensional view can buy pairs and view them through the stereoscope. Aerial photographs are also available in digital form as products called digital orthophoto quadrangles, or DOQ images. DOQ's have the same accurate measurements of distance and direction as do maps.