Digital Elevation Model (DEM)

Types of Elevation Model

The most common interpretation of the term digital elevation model, the one used in this entry, is as a regular grid of height values, sometimes synonymously referred to as a height field, especially in the domain of computer graphics. These grids can be stored and manipulated in exactly the same way as any other form of raster in a GIS. While the surface represented by a DEM occupies three dimensions, it is technically a two-dimensional surface, since any given location on the ground is associated with only one height value (see Figure 1). This means that a single DEM cannot be used to represent cliffs with overhangs or caves and tunnels. To distinguish DEMs from true three-dimensional data, DEM surfaces are sometimes referred to as being “2.5D.”

In geographic information science, a DEM can be used to represent any surface. Most commonly, this will be part of the earth's surface, such as part of a mountain range or coastal dune system. However DEMs have also been used to represent other planetary surfaces (the Martian surface measured by the Viking and Mars Global Surveyor orbiters being widely studied). The structure of DEMs can also be used to represent more abstract georeferenced surfaces, such as temperature, population density, and income.

Figure 1 Simple Raster Digital Elevation Model

Each raster cell represents a single height above a known datum (e.g., mean sea level). Right-hand image shows the same DEM in a 3D perspective view.

Unfortunately, there are a number of similar terms used to describe elevation models, some of which are used synonymously. The term digital terrain model (DTM) is sometimes used interchangeably with DEM, although it is usually restricted to models representing landscapes. A DTM can sometimes contain additional surface information, such as the location of local peaks and breaks in slope. The term digital surface model (DSM) describes a DEM that represents the upper surface of a landscape, including any vegetation, buildings, and other surface features (see Figure 2). This can be contrasted with a digital ground model (DGM), which represents the height of the land as if stripped of any surface vegetation or buildings.

DEMs are not the only form of surface model used in geographic information science. Triangulated irregular networks (TIN) also represent two-dimensional surfaces but store elevation values at irregular spatial intervals rather than a regular grid. Contour lines commonly depicted on topographic maps can also be processed in digital form to estimate elevation over a surface. DEMs have the advantage over both of these alternatives, in that they are more amenable to processing using the functionality common in most GIS.

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