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Page first created Monday, April 24, 2000
Page last modified Wednesday, May 23, 2001 8:10 PM
I made an imagemap index of the USGS 7.5' DEM archive so that the DEMs can be found by map location instead of only by name. The imagemaps are also linked to listings of DEMs by coordinate order just in case you wish to yse a text only interface (silly in this case).
There are other scale DEMs as well as the 7.5' ones with specifications as follows (from the USGS DEM Readme files):
The DEM data for 7.5-minute units correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series for all of the United States and its territories. Each 7.5-minute DEM is based on 30- by 30-meter data spacing with the Universal Transverse Mercator (UTM) projection. Each 7.5- by 7.5-minute block provides the same coverage as the standard USGS 7.5-minute map series.
The 7.5-minute Alaska DEM data correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series of Alaska by unit size. The unit sizes in Alaska vary depending on the latitudinal location of the unit. The 7.5- minute Alaska DEM data consist of a regular array of elevations referenced horizontally to the geographic (latitude/longitude) coordinate system of the North American 1927 Datum (NAD 27) or the North American 1983 Datum (NAD 83). The spacing between elevations along profiles is 1 arc second in latitude by 2 arc seconds of longitude.
(2x3 arc-second data spacing, latitude by longitude). Provides coverage similar to a 15-minute DEM, except that the longitudinal cell limits vary from 20 minutes at the southernmost latitude of Alaska to 36 minutes at the northern most latitude limits of Alaska. Coverage of one DEM will generally correspond to a 1:63,360-scale quadrangle.
The 1-degree DEM provides coverage in 1- by 1-degree blocks and is available for all of the contiguous United States, Hawaii, and most of Alaska. The basic elevation model is produced by the Defense Mapping Agency (DMA) using cartographic and photographic sources.
The 1-degree DEM consists of a regular array of elevations referenced horizontally on the geographic coordinate (latitude/longitude) system of the World Geodetic System 1984 Datum. Elevation data located on the degree lines (all four sides) correspond with the same profiles on adjoining DEM blocks.
Elevations are in meters relative to mean sea level. Spacing of the elevations along and between each p rofile is 3 arc-seconds with 1,201 elevations per profile. The only exception are DEMs in Alaska, where the spacing and number of elevations per profile varies depending on the latitudinal location of the DEM. Latitudes between 50 and 70 degrees north have spacings at 6 arc-seconds with 601 elevations per profile and latitudes greater than 70 degrees north have spacings at 9 arc-seconds with 401 elevations per profile.
The USGS EROS Data Center's GTOPO30 is a global digital elevation model (DEM) with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer). Here is an image map interface to the GTOPO30.
NIMA has an imagemap interface to their publicly available Digital Terrain Elevation Data (DTED®) Level 0 30 Arc Second Terrain Data of much of Earth's surface.
DTED® Level 0 elevation post spacing is 30 arc second (nominally one kilometer). In addition to this discrete elevation file, a separate binary file provides the minimum, maximum, and mean elevation values computed in 30 arc second square areas (organized by one degree cell). Finally, DTED¨ Level 0 contains the NIMA Digital Mean Elevation Data (DMED) providing minimum, maximum, and mean elevation values and standard deviation for each 15 minute by 15 minute area in a one degree cell.
Return to Mapping page or go up to DavidWoolsey.com.
Page first created Monday, April 24, 2000
Page last modified Wednesday, May 23, 2001 8:10 PM