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A digital elevation model (DEM) is a digital file consisting of terrain
elevations for ground positions at regularly spaced
horizontal intervals. The USGS produces five different digital elevation
products. Although all are identical in the
manner the data are structured, each varies in sampling interval, geographic
reference system, areas of coverage, and
accuracy; with the primary differing characteristic being the spacing, or
sampling interval, of the data. The five DEM
products:
Over the years, the USGS has collected digital elevation data using a
number of production strategies including manual
profiling from photogrammetric stereomodels; stereomodel digitizing of
contours; digitizing topographic map contour plates; converting
hypsographic and hydrographic tagged
vector files; and performing autocorrelation via automated photogrammetric systems.
Of these techniques, the derivation of DEM's from vector hypsographic and hydrographic data produces
the most accurate model, and is the preferred method.
Applications
DEM's may be used in the generation of three-dimensional graphics displaying terrain slope,
apsect
(direction of slope), and terrain profiles between selected points. At the
USGS, DEM's have been used in combination
with digital raster graphics (DRG's), digital line graphs (DLG's), and digital orthophoto quadrangles
(DOQ's) to both enhance the visual information for data
extraction and revision purposes and
to create aesthetically pleasing and dramatic hybrid digital images.
Non-graphic applications such as modeling terrain and
gravity data for use in the search for energy resources, calculating the
volume of proposed reservoirs, and determining
landslide probability have also been developed.
Data Format
A DEM file is organized into a series of three records, A, B, and C. The A
record contains information defining the
general characteristics of the DEM, including its name, boundaries, units of
measurement, minimum and maximum
elevations, number of B records, and projection parameters. Each B record
consists of an elevation profile with
associated header information, and the C record contains accuracy data.
Each file contains a single A and C record, while there is a separate
B record for each elevation profile. More detailed information about
the organization of a DEM file can be found
in the National Mapping Program Technical Instructions, Standards for Digital
Elevation Models and Digital
Elevations Models: Data User's Guide.
The USGS has plans to convert its DEM products to the Spatial
Data Transfer
Standard (SDTS) format and offer that data free over the Internet. SDTS
offers a mechanism for
transferring data between dissimilar computer systems and offers the
advantages of flexibility,
improved quality, and no loss of information. It is the transfer mechanism
for all Federal agencies.
Additional information about the SDTS, Federal Processing Standard 173,
or how to
participate in related development activities can be found on the
SDTS Home Page.
All USGS DEM's are available on 9-track, 8mm, and 3480 cartridge tape.
The 1-degree DEM's are available for free via
FTP (File Transfer Protocol). The 7.5-minute and 2-arc-second DEM's are also available for
distribution over the Internet via FTP. Saleable units
for DEM's are:
What is a DEM ?
Documentation and Information
Ordering Information
For product pricing and distribution inquires contact any
Earth Science Information Center or call 1-888-ASK-USGS.
U.S. Department of the Interior
U.S. Geological Survey
Rocky Mountain Mapping Center
Maintainer:webmaster@rockyweb.cr.usgs.gov
URL:http://rmmcweb.cr.usgs.gov
/elevation/dpi_dem.html
Last modified:09:28:56Mon18Jun2001
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