|Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact|
|Full name||USGS Digital Elevation Model (DEM) Format, 1998|
A digital elevation model (DEM) represents terrain elevations for ground positions at regularly spaced horizontal intervals. The USGS "native" DEM format was developed specifically for this category of data and dates from 1992. A DEM dataset is a single file comprising 1024-byte ASCII-encoded (text) blocks that fall into three record categories called A, B, and C. There is no cross-platform ambiguity since line ending control codes are not used, and all data, including numbers, is represented in readable text form. A raster file format, DEM files usually contain terrain elevations, but the file format can be used for representing any surface. The format, allows internal documentation of coordinate system, date of publication, etc. These values are referenced horizontally, either to a Universal Transverse Mercator (UTM) projection or to a geographic coordinate system
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 optional C record contains accuracy data. Each file contains a single A record and may contain a single C record, while there is a separate B record for each elevation profile.
|Production phase||DEM is a middle-state format used in the generation of three-dimensional graphics displaying terrain slope, aspect (direction of slope), and terrain profiles between selected points.|
|LC experience or existing holdings||The Library of Congress has a small number of DEM files in its collections.|
|LC preference||See the Recommended Formats Statement for the Library of Congress format preferences for GIS, Geospatial and Non-GIS Cartographic data.|
|Disclosure||Openly published specification by United States Geological Survey (USGS). Standards for Digital Elevation Models (link via Internet Archive) specifies the collection, processing, and quality control of digital elevation model (DEM) data intended for entry into the National Digital Cartographic Data Base (NDCDB).|
|Documentation||Published by USGS at Digital Elevation Model Standards. "Standards for Digital Elevation Models" was published in 1997/8 as four documents: an introduction; Part 1: General; Part 2: Specifications; and Part 3: Quality Control. No longer online as of May 2020. Links via Internet Archive.|
It is an open standard, and has been used throughout the world. It has been superseded by the USGS's own SDTS format but the format remains popular due to large numbers of legacy files, self-containment, relatively simple field structure and broad, mature software support.
As of 2006, the USGS no longer distributed elevation data in the DEM format, but, due to popular demand, USGS data was made available in the DEM format by other sources, e.g., WebGIS Terrain Data.
|Licensing and patents||No licensing concerns.|
|Self-documentation||The single type A record in a DEM file contains information describing the data in the type B records. The optional single type C record provides information on the accuracy of the data.|
|Technical protection considerations||None|
|GIS images and datasets|
Per the 1986 Standards for Digital Elevation Models, Part 2: Specifications (link via Internet Archives), two types of horizontal datums were used for DEM data distributed by the USGS, the civilian North American Datum (NAD) and military World Geodetic System (WGS). The NAD 27 datum was used to define positions on USGS topographic maps and 7.5-minute DEM's. The USGS NAD 83 for these applications. The WGS 72 is currently used to define positions for 1-degree NIMA DEM's and DTED's. The NIMA is converting these data to the new WGS 84. The NAD 83 and WGS 84 datums are being phased into the mapping community at different rates or where resources are available. For the conterminous United States, these new datums are considered to be functionally the same; however, the two have been defined separately because they were designed to serve different segments of the mapping community, primarily civilian and military. The following information will help clarify the relationship between these datums.
Standards for Digital Elevation Models, Part 3: Quality Control (link via Internet Archives) explains that the computer program, DEM VERIFY, is the standard software used to verify the logical and physical format as part of the data base entry procedure. Validation by this software applies to all DEM software systems of the USGS and other government agencies or contractors whose data are intended for submission to the NDCDB. In all cases where the integrity of the logical or physical DEM structure is in question, DEM VERIFY is the standard software for format verification.
|Support for GIS metadata||Two classes of metadata are provided for each product: textual metadata and spatial metadata. Textual metadata are XML files containing information specific to each downloadable dataset. Spatial metadata is delivered in a geopackage containing information about all source datasets. See Lidar Base Specification 2022 rev. A: Metadata for details.|
|Support for grids||DEM file formats store raster-based data set with a regular grid of elevations arranged by columns and row.|
|Beyond normal functionality||None.|
||See HEC-FIA Map Layer Formats.|
|Pronom PUID||See note.||PRONOM has no corresponding entry as of August 2022.|
|Wikidata Title ID||Q640492
|General||The USGS has produced 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.
DEMs may be used in the generation of three-dimensional graphics displaying terrain slope, aspect (direction of slope), and terrain profiles between selected points. At the USGS, DEMs 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.
Created by an act of Congress in 1879, the U.S. Geological Survey has evolved over the decades, matching its talent and knowledge to the progress of science and technology. The USGS is the sole science agency for the Department of the Interior. It is sought out by thousands of partners and customers for its natural science expertise and its vast earth and biological data holdings.
The DEM format originates from 1992. Originally, USGS DEM files were available on 9-track, 8mm, and 3480 cartridge tape. The 1-degree, 7.5-minute, and 2-arc-second DEM files were also available for distribution over the Internet via FTP. See USGS Digital Elevation Model Information from 2005.
The USGS initiated development of seamless elevation layers in the early 1990s to serve as one of the seven framework data themes of the National Spatial Data Infrastructure. The intent of a framework dataset was to collaboratively create geospatial data at a common standard among contributing agencies, and to create them in such a way as to provide a base layer for many applications and for use by a broad set of consumers. The first continental and global seamless datasets the USGS developed were at 1-kilometer resolution followed by regional prototypes in 1996. The first full coverage of conterminous United States (CONUS) was completed in 1997 and was based on a combination of approximately (~) 10-meter, ~30-meter, 2-arc-second, and 3-arc-second resolution source data. In 1999, the National Elevation Dataset was finished for CONUS using 10-meter and 30-meter data only.
Starting in 1995, the 7.5-minute USGS DEM data was converted to the newer SDTS format. From August 2001 until 2017, DEM datasets in SDTS format were available for download from GeoCommunity at the GIS Data Depot. Soon afterwards, other vendors provided similar services. See FGDC-Compliant USGS SDTS 7.5' DEMs available online.
Starting in 2006, USGS no longer distributed elevation data in the DEM format. However, USGS elevation data in the DEM format has continued to be available from other sources, e.g., WebGIS Terrain Data. Other entities continued to issue elevation data in the USGS "native" DEM format described here.
See Free Digital Elevation Model (DEM) and Free Satellite Imagery Download Links for a discussion of the transition away from the DEM format by USGS and links to sites that support the ongoing demand for elevation data in the DEM format.