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DATA DISTRIBUTION CENTERS 
A data center was established by the EROS (Earth Resources Observa- 
tion Systems) Program of the U.S. Department of the Interior at 
Sioux Falls, South Dakota, in October 1971 to serve as a principal 
outlet for NASA space and aircraft remote-sensor data and 
Department of the Interior aerial photography. The EROS Data Center 
moved into its permanent facility, the Karl E. Mundt Federal Building, 
25 km north-northeast of Sioux Falls, in the fall of 1973. At the 
end of 1975, holdings were 6 1/3 million frames of aerial and space 
photographs and images, including about 700,000 Landsat frames. In 
addition to distributing data, the Center provides training and 
assistance in uses of the data, and conducts demonstration programs 
and projects to develop advanced techniques for applying remote-sensor 
data to specific resource and environmental problems. 
Landsat imagery and U.S. Department of Agriculture aerial photography 
are distributed by the Department of Agriculture photographic labora- 
tory in Salt Lake City, Utah.  Landsat, NOAA/NESS, and DMSP imagery 
and U.S. Department of Commerce aerial photography are distributed 
by the NOAA/NESS photographic laboratory in Suitland, Maryland. 
The National Cartographic Information Center of the U.S. Department 
of the Interior, Geological Survey, was organized in 1975. Among its 
functions is a centralized facility for ordering of all United States 
government-acquired aerial photography. 
IMAGE INTERPRETATION TECHNIQUES 
Image interpretation is a logical extension of photointerpretation 
to images produced by instruments other than cameras in which electro- 
magnetic radiation (EMR) reflected from the scene is recorded. These 
instruments produce imagery by indirect means; the EMR is directed 
by optical elements onto a detector, whose output is fed into electronic 
circuitry. The output of the electronic circuitry may be recorded on 
magnetic tape in either analog or digital form, transmitted to distant 
antennas, and further processed prior to producing an image. 
The widespread availability of multiband imagery, especially from 
instruments whose outputs can be digitized, has led to the development 
of new interpretation techniques, which may be grouped into three 
categories: passive, interactive, and automated classification. 
Passive techniques are those by which the data are enhanced before 
and/or during conversion to image form, to provide the interpreter 
with & product of increased interpretability. Interactive techniques 
permit the interpreter to manipulate the data while performing his 
interpretation, and see the results of his manipulation on a display 
device or devices. Automated classification is the delineation (and 
portrayal) by digital or analog methods of particular features with 
specific and different reflectances in various spectral bands of 
data (Iindenlaub, 1973).