A large amount of remotely sensed data has been acquired over many areas of the world, but it is difficult for an individual to identify sources of existing data and to determine coverage, imagery characteristics, availability, and costs. Many federal and state agencies do not distribute descriptions of remotely sensed data holdings to the public. A comprehensive survey conducted by J. R. May in 1978 identifies many sources of these data in federal and state organizations.

The U.S. Geological Survey's EROS Data Center (EDC) is the national distribution facility for LANDSAT and other remotely sensed data products. Data stored at EDC at the end of 1978 included over 1,200,000 frames of LANDSAT imagery; over 56,000 images from Skylab, Apollo, and Gemini; over 1,400,000 images from the NASA research aircraft programs; and over 3,300,000 aerial mapping photographs from the U.S. Department of the Interior, U.S. Department of Commerce, and other federal agencies. Availability and

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characteristics of data stored at EDC can be determined by requesting a geographic computer search using latitude and longitude of the area of interest.

A new all-digital system for handling and processing LANDSAT data is in operation at EDC. The data are radiometrically corrected for detector gain and offset and geometrically corrected to a Space Oblique Mercator (SOM) map projection, using cubic convolution resampling techniques. Upon request, the user can obtain data which are corrected to the Universal Transverse Mercator (UTM) or Polar Stereographic (above 65° lat.) map projections with the nearest neighbor resampling technique. The user may also request geometrically uncorrected high-density digital tapes. When placing an order for LANDSAT image products, the user may select or omit contrast and edge enhancements. Film and digital tape products from the LANDSAT multispectral scanner system (MSS) and return beam vidicon (RBV camera are available in a variety of formats and scales.

LANDSAT data over areas outside the United States are available at EDC; these data are acquired by receiving stations in Canada, Brazil, Italy, and Iran and are also available from these countries. Scale, format, and prices of LANDSAT products from foreign receiving stations are similar to those distributed by EDC.

Other federal and state agencies duplicate aerial photographs for users on request. Scale, area coverage, and film type vary. The most common aerial photography is panchromatic at a scale of about 1:20,000 and in a 9 × 9-in. format. Some color and color infrared photographs also may be available.

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