Digital elevation model
n. A digital-based numerical representation of the ground surface made up of equal-sized, regularly spaced grid cells with an elevation value for each cell.
Elevation or geometric height is used when referring to points on the earth’s surface, while altitude is used for points above the earth’s surface.
Digital elevation model (DEM) data consist of arrays of regularly spaced elevation values referenced horizontally either to a universal transverse mercator (UTM) projection or to a geographic coordinate system. The grid cells are spaced at regular intervals along south to north profiles that are ordered from west to east.
In a geographic information system (GIS), digital elevation models are commonly used to represent the surface (topography) of a place through a raster (grid) data set of elevations. Spatial data layers representing elevation, slope, and aspect can be developed within a GIS using information from a DEM. This information is required for spatial fire modeling systems such as FARSITE or FlamMap.
In the past, digital elevation data were available based on a 7.5-minute topographic quadrangle extent. To create a digital representation of elevation for a large area required the user to download multiple files, merge them together, and account for data issues arising from the merging process. Currently, the U.S. Geological Survey has numerous seamless digital elevation data products available that can be accessed via the Internet (see http://edc.usgs.gov/products/elevation.html). See AMS (2000), USGS (2000), and Whiteman (2000) for further discussion.