2-Arc-Second Digital Elevation Models

The 2-arc-second digital elevation model (DEM), formerly known as the 30-minute DEM, is a standard product of the National Mapping Program of the U.S. Geological Survey. These DEM's are produced as a by-product of a cooperative production partnership with the Defense Mapping Agency, and also under cooperative partnerships with the U.S . Forest Service and the Bureau of Land Management.

Product Description

A 2-arc-second DEM has the following characteristics:

The product consists of a regular array of elevation referenced horizontally to the geographic (latitude/longitude) coordinate system of NAD 27 or NAD 83.

The unit of coverage is a 30- by 30-minute block. Saleable units are four 15-minute DEM's covering a 30- by 30-minute area (the areas corresponding to the east or west half of a USGS 1:100,000-scale series topographic quadrangle map). Elevation data on the integer minute lines (all four sides) correspond to the same profiles on the surrounding eight blocks.

Elevations are in meters or feet relative to NGVD 29 in the continental U.S. and local mean sea level in Hawaii and Puerto Rico.

The data are ordered from the south to the north in profiles that are ordered from west to east.

Spacing of the elevations along each profile is 2-arc-seconds. The first and last data points are at the integer 15 minutes of latitude. A 15-minute profile will therefore contain 451 elevations.

The USGS uses two processes to collect the digital elevation data for production of 2-arc-second DEM's: (1) DLG hypsographic data derived from standard USGS topographic quadrangle series maps (1:24,000- to 1:100,000-scale); (2) resampling from DEM's with a source spacing equal to or less than 2-arc-second sampling interval (if the data is resampled from preexisting DEMs, it is arbitrarily archived as level 1). DEM data for this series are tested according to level 2 accuracy specifications.

The accuracy of the DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:100,000. The plotting of contours from 2-arc-second DEM data at scales larger than 1:100,000 , or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, will lead to less reliable results.

Production Status

The 2-arc-second DEM program covers the conterminous United States and Hawaii. There is approximately 35 percent completed coverage at present, (status graphic) with full coverage expected in 2-3 years.

Sample Images

Sample images generated from a portion of the Blanco Peak, CO, 2-arc-second DEM:

U.S. Department of the Interior
U.S. Geological Survey
Rocky Mountain Mapping Center
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URL:http://rockyweb.cr.usgs.gov /elevation/2arcinfo.html
Last modified:09:29:15Mon18Jun2001
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