The USGS Digital Elevation Model (DEM) data files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. These digital cartographic/geographic data files are produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program and are sold in 7.5-minute, 15-minute, 2-arc-second (also known as 30-minute), and 1-degree units. The 7.5- and 15-minute DEMs are included in the large scale category while 2-arc-second DEMs fall within the intermediate scale category and 1-degree DEMs fall within the small scale category.
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.
The UTM-based 7.5-minute DEM data are available for much of the contiguous United States, Hawaii, and Puerto Rico. Data availability for the contiguous United States may be viewed on a status coverage map. The geographic-based 15-minute Alaska DEM data are available for most of Alaska.
The 7.5-minute DEM data are produced in 7.5- by 7.5-minute blocks either from digitized cartographic map contour overlays or from scanned National Aerial Photography Program (NAPP) photographs.
Four processes have been used to generate DEM data for 7.5-minute units. The first three processes have been discontinued:
The Gestalt Photo Mapper II (GPM2) is an automated photogrammetric system designed to produce orthophotos, digital terrain data, and contours in subunits known as patches.
Manual profiling from photogrammetric stereomodels uses stereoplotters equipped with three-axis electronic digital profile recording modules for scanning stereomodels along successive terrain profiles.
Elevations are recorded by stereomodel digitizing of contours, in which digital contours are acquired on stereoplotters equipped with three-axis digital recording modules.
Interpolation from digital line graph (DLG) hypsographic and hydrographic data. All data now being generated use this process.
Each 7.5-minute unit of DEM coverage (based on the 7.5-minute quadrangle) consists of a regular array of elevations referenced horizontally in the UTM projection coordinate system. Elevation units are in meters or feet relative to National Geodetic Vertical Datum of 1929 (NGVD 29) in the continental U.S. and local mean sea level in Hawaii and Puerto Rico. The data are ordered from south to north in profiles that are ordered from west to east.
These horizontally referenced data may be NAD 27, NAD 83, Old Hawaiian Datum (OHD), or Puerto Rico Datum (PRD) of 1940.
The 7.5-minute DEM data are stored as profiles in which the spacing of the elevations along and between each profile is 30 meters. The number of elevations in a profile will vary because of the variable angle between the quadrangle's geographic boundary (neatline) and the UTM coordinate system. DEM data of low-relief terrain or generated from contour maps with intervals of 10 feet or less are recorded in feet while DEM data of moderate to high-relief terrain or generated from maps with terrain contour intervals greater than 10 feet are generally recorded in meters.
The vertical accuracy of 7.5-minute DEMs is equal to or better than 15 meters. A minimum of 28 test points per DEM is required (20 interior points and 8 edge points). The accuracy of the 7.5-minute 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 1:24,000 scale.
A digital elevation model is composed of integer values representing a gridded form of a topographic map hypsography overlay.
Additional information on DEM specifications can be found in the USGS National Mapping Program's Standards for Digital Elevation Model (DEMs) and in the USGS publication titled "Digital Elevation Models -- Data Users Guide 5."
DEM data are organized in three classification levels. Level-1 DEMs are elevation data sets in a standardized format. The intent is to reserve this level for 7.5-minute DEMs which are created by scanning National High Altitude Photography (NHAP)/NAPP photography. A vertical RMSE of 7 meters is the desired accuracy standard. A RMSE of 15 meters is the maximum permitted.
Level-2 DEMs are elevation data sets that have been processed or smoothed for consistency and edited to remove identifiable systematic errors. DEM data derived from hyposographic and hydrographic data digitizing, either photogrammetrically or from existing maps, are entered into the Level-2 category. A RMSE of one-half contour interval is the maximum permitted.
Level-3 DEMs are derived from DLG data by incorporating selected elements from both hypsography (contours, spot elevations) and hydrography (lakes, shorelines, drainage). A RMSE of one-third of the contour interval is the maximum permitted.
Digital Elevation Model Caveats
The majority of the 7.5-minute DEMs produced to date are categorized as Level-1 DEMs.
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 DEMs.
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.