Digital Elevation Models – DEMs
Digital Elevation Models – DEMs are data files that contain the Mean Sea Level (MSL) elevation of the terrain over a specified area, usually at a fixed grid interval over the “Bare Earth”. The intervals between each of the grid points will always be referenced to some horizontal geodetic reference system and regional vertical datum. This is usually either latitude-longitude or UTM (Universal Transverse Mercator) coordinate systems and MSL elevation.
Within the same project area, a Digital Surface Model (DSM) represents the MSL elevations of the reflective surfaces of trees, buildings, and other features elevated above the “Bare Earth”.
3m Digital Surface Model (DSM) from Stereo IKONOS Satellite Image Data
Image copyright © MAXAR/Processed by Satellite Imaging Corporation. All rights reserved.
The closer together the grid points are located, the more detailed the information will be in the file. The details of the peaks and valleys in the terrain will be better modeled with small grid spacing than when the grid intervals are very large. Elevations other than at the specific grid point locations are not contained in the file. As a result peak points and valley points not coincident with the grid will not be recorded in the file.
DEMs have several meanings and are not always understood correctly or misinterpreted due to the surface it represents or the geographic location where the DEM data is being used. As used in most of the western hemisphere a DEM has “Bare Earth” z-values at regularly spaced intervals in x (Easting’s) and y (Northing’s). In some countries, the x and y orientation is opposite from the general use with “x” being Northing and “y” Easting. Because of many different horizontal and vertical geodetic reference systems available around the world, the necessity to clearly define the 3D geodetic reference system to be utilized is critical to the success of a project to obtain the mapping standards defined.
To represent the earth’s surfaces and bathymetry/hydrographic features in a 2D or 3D Geographic Information Systems (GIS) or CAD computer environment relative to existing geodetic horizontal and vertical datum the reference surfaces have to be clearly defined.