1. 7.5-Minute DEMs correspond to the USGS 1:24,000- and 1:25,000-scale topographic quadrangle maps, and are available for all of the U.S. and its territories. They are cast to the Universal Transverse Mercator (UTM) projection system and are referenced to either the North American Datum (NAD) of 1927 (NAD27) or the NAD of 1983 (NAD83). Most files will have a grid spacing of 30 meters, but 10-meter grids are also available for some locations. The average file size of a 30-meter DEM is 1.1 megabytes and 9.9 megabytes for a 10-meter DEM. They are available in Native and Spatial Data Transfer Standard (SDTS) format. Native format is distributed on CD-ROM, DVD, 8-mm tape, and File Transfer Protocol (FTP). SDTS is distributed via FTP download.

2. 7.5-Minute Alaska DEMs correspond to the USGS 1:24,000- and 1:25,000-scale topographic map series for Alaska. The exact area of coverage will vary according to latitude. They are expressed in geographic coordinates (latitude/longitude) and are referenced to either NAD27 or NAD83. Grid spacing is 1 arc-second of latitude by 2 arc-seconds of longitude. Files are available in Native format and are distributed on CD-ROM, DVD, 8-mm tape, and FTP.

3. 15-Minute Alaska DEMs correspond to the USGS 1:63,360-scale topographic map series for Alaska. The exact area of coverage will vary according to latitude. They are expressed in geographic coordinates (latitude/longitude) and are referenced to either NAD27 or NAD83. Grid spacing is 2 arc-seconds of latitude by 3 arc-seconds of longitude. Files are available in Native format and are distributed on CD-ROM, DVD, 8-mm tape, and FTP.

4. 30-Minute DEMs correspond to the east half or west half of the USGS 30-minute by 60-minute topographic quadrangle map, and are available for the conterminous U.S. and Hawaii. They are expressed in geographic coordinates (latitude/longitude) and are referenced to either NAD27 or NAD83. Grid spacing is 2 arc-seconds. Files are available in Native format and are distributed on CD-ROM, DVD, 8-mm tape, and FTP.

5. 1-Degree DEMs correspond to the 3 arc-second (or 1:250,000-scale) USGS topographic map series, and are available for all of the contiguous U.S. and most of Alaska. They are expressed in geographic coordinates (latitude/longitude). Most are referenced to the World Geodetic Survey (WGS) system of 1972 (WGS72), but some are referenced to the WGS system of 1984 (WGS84). Grid spacing is 3 arc-seconds. Files are available in Native format and are distributed via FTP download.

Additional Information:

DEM Fact Sheet
DEM Standards
DEM Guide
7.5-Minute DEM Abstract
15-Minute Alaska DEM Abstract
30-Minute DEM Abstract