A Digital Elevation Model consists of a sampled array of elevations for ground positions that are normally at regularly spaced intervals. The U.S. Geological Survey produces five primary types of digital elevation model data. They are:
7.5-minute DEM (30- x 30-m data spacing, cast on Universal Transverse Mercator (UTM)
projection or 1- x 1-arc-second data spacing). Provides coverage in 7.5- x 7.5-minute
blocks. Each product provides the same coverage as a standard USGS 7.5-minute map series
quadrangle.
-- Coverage: Contiguous United States, Hawaii, and Puerto Rico.
1-degree DEM (3- x 3-arc-second data spacing). Provides coverage in 1- x 1-degree
blocks. Two products (three in some regions of Alaska) provide the same coverage as a
standard USGS 1-x 2-degree map series quadrangle. The basic elevation model is produced by
or for the Defense Mapping Agency (DMA), but is distributed by USGS in the DEM data record
format.
-- Coverage: United States.
30-minute DEM (2- x 2-arc-second data spacing). Consists of four 15- x 15-minute DEM
blocks. Two 30-minute DEMs provide the same coverage as a standard USGS 30- x 60-minute
map series quadrangle. Saleable units will be 30- x 30-minute blocks, that is, four 15- x
15-minute DEMs representing one half of a 1:100,000-scale map.
-- Coverage: Contiguous United States, Hawaii.
15-minute Alaska DEM (2- x 3-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.
7.5-minute Alaska DEM (1- x 2-arc-second data spacing, latitude by longitude). Provides coverage similar to a 7.5-minute DEM, except that the longitudinal cell limits vary from 10 minutes at the southernmost latitude of Alaska to 18 minutes at the northernmost latitude limits of Alaska.
The UTM-based DEM's (7.5-minute DEM) and the geographic-based DEM's (30-minute, Alaska, and 1-degree DEM's) are identical in logical data structure but differ in sampling interval, geographic reference system, areas covered, and horizontal and vertical accuracy. Knowledge of all of these properties is essential to ensure that the user does not exceed the useful limits of the data for required applications. The 7.5-minute UTM DEM's are available for selected quadrangles, which are indicated on a status graphic published biannually by USGS. The 1-degree DEM's (1:250,000 scale) are available for all of the contiguous United States, Hawaii, and portions of Alaska, Puerto Rico, and the Virgin Islands. Many of the original 1-degree DEM's are being replaced with more accurate digital models through a cooperative regridding project with NIMA, scheduled for completion in 1995. As they become available, these 1-degree DEM's will replace their corresponding product. The 30-minute and Alaska DEM's are new DEM series and are available on a limited basis as projects are completed.
Digital Elevation Data Sources and Documentation
DEM overview
- documentation for all scales
1-Degree DEM data
available on-line and additional documentation for all types of DEM data.
Only 1-degree DEM data is available free on line. 1-Degree data is available for the entire country. 7.5 minute data is not yet available for the entire country, but is a priority product and national coverage is planned. Status maps are available at this site.
Data User's Guide for DEM data. This is a large document that discusses in detail the technical aspects of DEM data. Go to the DEM subdirectory for the most current documents. A copy of the document is available here also in MS Word format, zipped with PKZip, and split in two parts; the basic contents and the appendices. The appendices cover the format of the DEM records, as well as considerable information on map projection parameters and UTM and State Plane zones. Click here to get the main document or the appendices. These versions were current as of February, 1998.
Tips for Using DEM Data
DEM data by itself, unmanipulated, is not extremely useful. It does make nice graphics (see the samples in the on-line DEM overview documentation above), but needs to be combined with other data layers or manipulated and used in models to reach it's full potential. DEM data is raster in format, unlike the USGS DLG data which is vector. Raster data is much more useful for analytical models (watershed analysis, erosion potential, etc), but requires different tools than vector data. DEM data cannot be read into ArcView, for example, unless the Spatial Analyst Extension is loaded. Arc/Info can read the files and do some data transformations, but Arc/Grid is needed to do extensive modeling.
With a tool such as the Spatial Analyst, one DEM file can be used to create slope, aspect, contour, and viewshed type files. These can be combined with other data to create numerous models (EG soil erosion potential or habitat suitability for plants or animals). They can also be combined with Digital Orthophoto Quads (DOQ's) and used as excellent background layers for displaying GIS information; the DOQ supplies the picture and the DEM adds the topographic relief. DEM's are required in the production of DOQ's, and are an additional benefit of DOQ production.
For additional information regarding this Web page, contact Deb Southworth Green , in the Division of Information Resources and Technology Management, at Deb_Green@fws.gov
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Keywords=GIS, data, DEM, elevation
Last Modified January 03, 2001 06:51 AM