The term DSM is generally applied regardless of whether the data are in gridded format (as in the DEM defined above) or mass point format (as in the DTM defined above). Digital Surface Model (DSM)Ī digital surface model (DSM) includes features above the ground, such as buildings and vegetation, and is used to distinguish a bare-earth elevation model from a non-bare-earth elevation model. A DTM is often more expensive and time consuming to collect than a DEM, but is considered technically superior for most engineering analyses because it retains natural features of the terrain. In addition to mass points, the DTM data structure often incorporates breaklines (further defined below) to retain abrupt linear features in the model. Fewer points can describe very flat or evenly sloping ground more points can be captured to describe very complicated terrain. Direct observations of elevation at a particular location can be incorporated without interpolation, and the density of points can be adjusted so as best to characterize the actual terrain. Digital Terrain Model (DTM)Ī digital terrain model (DTM) data structure is also made up of x,y points with z-values representing elevations, but unlike the DEM, these may be irregularly or randomly spaced mass points. The DEM, because it is a raster data structure similar to a digital image, is an efficient format for storage, analysis, rendering, and visualization. Furthermore, in a DEM, it is unlikely that the sharp edge of the feature will be represented correctly in the terrain model. Linear features, such as streams, drainage ditches, ridges, and roads are often lost in a DEM if the grid spacing is larger than the dimensions of the feature. Lidar, for example, yields a dense set of irregularly spaced points interpolation to a grid requires using one of many possible interpolation algorithms, which produce varying results. The observations, or direct measurements, of elevation that comprise the DEM are almost never actually captured on a regular grid therefore, the elevation for any given point in the grid is normally interpolated from other forms of source data. The elevation of lakes and rivers in a DEM implies the height of the water surface based on elevation of the exposed shoreline. The z-values in a DEM represent the height of the terrain, relative to a specific vertical datum and void of vegetation or manmade structures such as buildings, bridges, walls, et cetera. Digital Elevation Model (DEM)Ī digital elevation model (DEM) contains elevations at points arranged in a raster data structure, a regularly spaced x, y grid, where the intervals of Δx and Δy are normally in linear units (feet or meters) or geographic units (degrees or fractions of degrees of latitude or longitude). This unit provides definitions for the types of terrain models most commonly encountered for mapping and GIS applications. A terrain surface can be characterized in many ways depending on the interest and perspective of the end user.
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