This information about aerial photographs refers to the products of the National Aerial Photography Program (NAPP), an interagency Federal effort coordinated by the U.S. Geological Survey (USGS). The NAPP is designed to cover all the lower 48 States every 5 to 7 years with a new set of aerial photographs. The USGS uses the photographs largely for revising its famous topographic maps; the U.S. Forest Service, for monitoring forest conditions; the Natural Resources Conservation Service, for looking at soils and land cover; and so on. The photographs, in fact, are helpful for a great many purposes -- mapmakers, land resource managers, earth scientists, life scientists, engineers, urban planners, and recreational users all purchase large numbers of these photographs. Many members of the general public also use them simply for a novel view of a familiar landscape, such as a park, farm, or recreational area.
At its EROS Data Center, near Sioux Falls, S. Dak., the USGS houses more than 6 million aerial photographs of the United States, some dating from the 1940's. In addition to USGS aerial photography, the Center also holds photographs from other Federal agencies, bringing the entire collection to more than 8 million pictures.
The earlier photographs come from a variety of programs and vary in scale and other ways. Aside from pure historical or nostalgic interest, they are useful alongside more recent images to document land-surface changes.
Making a map from an aerial photograph is a time-consuming process that involves deleting the tones, texture, and detail not needed to convey the themes of the map. Other information--important things such as place names--is added in the mapmaking process. So aerial photographs actually have more information of some kinds than do the maps that are made from them. But for most purposes, they are best used in conjunction with maps.
Most NAPP photographs are black and white, but some are color-infrared, the better to show certain differences in plant life. They are shot from airplanes flying at a constant altitude of 20,000 feet, and each is shot straight down from the plane. Each 9- by 9-inch print covers an area about 5 miles square at an approximate scale of 1:40,000, where an inch represents about 0.6 mile on the ground. (With enlargement, of course, the scale is changed, so that an 18-inch print has a scale of 1:20,000.) 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.
Interpreting an aerial photograph, as distinguished from merely looking at one, requires a little skill. People spend years becoming proficient photointerpreters, and the amateur or casual user of these images is wise to spend a few minutes learning to distinguish a small cloud shadow from a small lake, an air force base from a civilian airport, or a stand of young timber from a grassy hillside.
Aerial photographs contain a lot of information, so different users examine them for different purposes. A consulting civil engineer may be looking for anomalies in drainage patterns, for example. A geologist may examine landforms to make deductions about the subsurface. A duck hunter may look for stock ponds that don't show up on topographic maps. So the engineer or geologist would use two photographs and a stereoscope, while the hunter can look at a single photograph unaided as he spreads it across the hood of his truck. In both cases, however, the users are likely to examine the photographs and USGS topographic maps together, perhaps alternating between the two to connect place names and road numbers with unidentified features in the photographs.
NAPP photographs may be obtained from a selected list of USGS Business Partners.
The NAPP inventory can be searched via EarthExplorer, using Photo Finder, where it is possible to place orders online.
NAPP photographs are available as paper prints, film negatives and positives, and diapositives.
Additional information is available through the World Wide Web or from USGS EROS Data Center Customer Services at 605-594-6151. The e-mail address is custserv@edcmail.cr.usgs.gov.