A Digital Elevation Model (DEM), consists of a sampled array of elevations for ground positions that are normally at regularly spaced intervals. The 1-Degree DEM (3- by 3-arc-second data spacing) provides coverage in 1- by 1-degree blocks for all of the contiguous United States, Hawaii, and limited portions of Alaska. The basic elevation model is produced by or for the Defense Mapping Agency (DMA), but is distributed by the USGS, EROS Data Center, in the DEM data record format. In reformatting the product the USGS does not change the basic elevation information. 1- degree DEM's are also referred to as "3-arc second" or "1:250,000 scale" DEM data.
Extent of Coverage
1-Degree DEM data are available for all of the contiguous United States, Hawaii, and most of Alaska.
Acquisition
Processing Steps
The majority of the 1-degree Digital Elevation Models are produced by DMA from cartographic and photographic sources.
Elevation data from cartographic sources are collected from any map series 7.5 minute through 1 degree (1:24,000 scale through 1:250,000 scale). The topographic features (contours, drain lines, ridge lines, lakes, and spot elevations) are first digitized and then processed into the required matrix form and interval spacing.
Elevation data from photographic sources are collected by using manual and automated correlation techniques. Elevations along a profile (see profile figure)are collected at 80 to 100 percent of the eventual point spacing. The raw elevations are weighted with additional information such as drain, ridge, water, and spot heights during the interpolation process in which final elevations are determined for the required matrix form and interval spacing.
The digital elevation models distributed within the Department of Defense cover 1- x 1-degree blocks and are called Digital Terrain Elevation Data Level 1 (DTED-1). In reformatting the DMA product to create the DEM's, the USGS restructures the header records and data but does not change the basic elevation information.
The DMA 1-degree DTED-1 data and USGS-distributed 1-degree DEM's are gridded by using the World Geodetic System 1972 (WGS 72) or World Geodetic System 1984 (WGS 84). All 1-degree DEM's are being converted to WGS84 and will be available from the USGS in 1992.
Data Characteristics
The 1-degree DEM consists of a regular array of elevations referenced horizontally on the geographic (latitude/ longitude) coordinate system of the WGS 72 (to be converted to WGS 84). The information content is approximately equivalent to that which can be derived from contour information represented on 1:250,000 scale maps.
The unit of coverage is a 1- x 1- degree block. Elevation data on the integer degree lines (all four sides) overlap with the corresponding profiles on the surrounding eight blocks.
Elevations are in meters relative to National Geodetic Vertical Datum of 1929 (NGVD 29) in the continental U.S. and local mean sea level in Hawaii. DEM accuracy information is provided in the APPENDIX.
Spatial Resolution
Spacing of the elevations along and between each profile is 3-arc seconds with 1,201 elevations per profile. The exception is DEM data 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.
Data Organization
A DEM file is organized into three logical records: types A, B, & C. The type A record contains information defining the general characteristics of the DEM, including DEM name, boundaries, units of measurement, minimum and maximum elevations, number of type B records, and projection parameters. There is only one type A record per DEM file. The type B record contains profiles of elevation data and associated header information. There is a type B record for each profile. The type C record contains statistics on the accuracy of the data. An example of a record format can be found in the appendix.
Data Availability
Procedures for Obtaining Data
The data are available (at no cost) through an Internet anonymous File Transfer Protocol (FTP) account at the EDC.
The data are also available (at cost of reproduction) on magnetic tape and can be ordered by 1:250,000 quad name with the east or west portion specified (e.g. Yellowstone East). Files can also be ordered by southeast latitude and longi tude of the 1-degree block (east or west portion of quad area).
FTP Instructions
To access the account:
ÑFTP to edcftp.cr.usgs.gov
ÑEnter "anonymous" at the Name prompt.
ÑEnter your complete e-mail address at the Password prompt.
ÑChange (cd) to the "pub/data/DEM/250" subdirectory.
ÑSet the file transfer mode to binary by typing the word "binary".
ÑUse the "get" and "mget" commands to download the 00README or data files.
ÑThe 00README file located under the "/pub/data/DEM/250" subdirectory contains an explanation of the directory structure and instructions for uncompressing data files.
The files are also available sorted by state, or can be selected from an index map.
To place orders and obtain additional information regarding technical details and prices, contact:
Customer Services, EROS Data Center
Products and Services
DEM data are available from and anonymous FTP account (see FTP instructions above) or from the EROS Data Center written as ANSI-standard ASCII characters in fixed length blocked record format on unlabeled or ANSI labeled magnetic tape. The logical record length is 1,024 bytes with a block size of 4,096 bytes (four logical records per physical record). The logical record is padded with blanks if necessary to fill to the end of the logical record (records are not "spanned").
Applications and Related Data Sets
The DEM files have been used in the generation of graphics such as isometric projections displaying slope, direction of slope (aspect), and terrain profiles between designated points. They may also be combined with other data types such as stream locations and weather data to assist in forest fire control, or with remote sensing data to classify vegetation. Other applications such as reduction of magnetics or gravity data, determining the volume of proposed reservoirs, calculating the amount of cut and fill materials, and to assist in determining landslide probability have also been developed.
The accuracy of the 1-Degree DEM product, 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 1:250,000. The plotting of contours from the 1-degree DEM at scales larger than 1:250,000, or reliance on the elevation heights without incorporating the National Map Accuracy Standard (NMAS) horizontal error tolerance, will lead to less reliable results. The USGS also distributes 7.5-minute, 15-minute and 30-minute Digital Elevation Models. Further information regarding these additional data sets may be obtained by contacting: Earth Science Information Centers (ESICs)
References
Digital Elevation Models, National Mapping Program Technical Instructions, Data Users Guide 5, U.S. Geological Survey, Second Printing (Revised), Reston, Virginia, 1990.
Appendix
ÑType A Record
ÑType B Record
ÑType C Record
ÑDEM Accuracy
DEM Accuracy
General Information
The accuracy of a DEM is dependent upon its source and the spatial resolution, that is grid spacing, of the data profiles. One factor influencing DEM accuracy is source data scale and resolution. A dependency exists between the scale of the source materials and the level of grid refinement possible. The source resolution is also a factor in determining the level of content that may be extracted during digitization. For example, 1:250,000-scale topographic maps are the primary source of 1-degree DEM's.
Another factor is the horizontal and vertical dimension of the DEM. Horizontal accuracy of DEM data is dependent upon the horizontal spacing of the elevation matrix. Within a standard DEM, most terrain features are generalized by being reduced to grid nodes spaced at regular intersections in the horizontal plane. This generalization reduces the ability to recover positions of specific features less than the internal spacing during testing and results in a de facto filtering or smoothing of the surface during gridding.
Vertical accuracy of DEM data is dependent upon the spatial resolution (horizontal grid spacing), quality of the source data, collection and processing procedures, and digitizing systems. As with horizontal accuracy, the entire process, beginning with project authorization, compilation of the source data sets, and the final gridding process, must satisfy accuracy criteria customarily applied to each system. Each source data set must qualify to be used in the next step of the process. Errors have the effect of compounding for each step of the process. Production personnel are directed to account for each production step leading to the final DEM.
1-Degree DEM Accuracy
The broad DMA production objective for a 1-degree DTED-1 is to satisfy an absolute horizontal accuracy (feature to datum) of 130 m, circular error at 90-percent probability; and an absolute vertical accuracy (feature to mean sea level) of + or - 30m, linear error at 90-percent probability. The relative horizontal and vertical accuracy (feature to feature on the surface of the elevation model), although not specified, will in many cases conform to the actual hypsographic features with higher integrity than indicated by the absolute accuracy.
