Arrays of topographic elevations at regularly spaced 30 meter (approximately 100 feet) intervals that correspond in coverage to standard 1:24,000-scale 7.5-minute quadrangles; produced by the U.S. Geological Survey and distributed in tape format.
(DEM) A raster file containing elevations at regularly-spaced surface coordinates over an area. Specialized computer software can interpret this data and create a three-domentional rendering of the surface, or analyze the terrain in various ways. See Also: raster More information on DEMs.
A data file of a topographic surface arranged as a set of x,y,z coordinates where z represents surface elevation. It is the digital equivalent of the elevation data portrayed on a topographic base map.
A digital cartographic/geographic dataset of elevations (topography) in xyz coordinates. The terrain elevations for ground positions are sampled at regularly-spaced horizontal intervals. DEMs are derived from contour lines and/or photogrammetric methods using USGS 7.5-minute, 15-minute, 2-arc-second (30- by 60-minute), and 1-degree (1:250,000-scale) topographic quadrangle maps. Digital elevation models are typically used to represent terrain relief.
A digital representation of a topographic surface. Elevation values may be stored in a regular grid of values, one for each cell, a regular lattice of points or in a 'triangulated irregular network' (TIN) of points.
A regular pattern of ground elevation observations maintained in digital form. These can be used to generate contour lines and Triangulated Irregular Networks (TINs) which look like wire frame representations of the terrain. They are also used in the process of orthogonalizing air photos.
A 3-D model of the Earth's surface represented by masspoints and breaklines. Less rigorous (not as densely collected) than a DTM. A DEM does not support contour modeling and is primarily used for orthorectification of aerial photography to the Earth's surface.
A digital representation of a continuous variable over a two- dimensional surface by a regular array of z values referenced to a common datum. Digital elevation models are typically used to represent terrain relief. Also referred to as 'digital terrain model' (DTM).
A digital elevation model (DEM) is a type of Digital terrain model, recording a topographical (geomorphometric) representation of the terrain of the Earth or another surface in digital format. DEM's record altitude in a raster format. That is, the map will normally divide the area into rectangular pixels and store the elevation of each pixel.