More Information about DEMs

•  USGS Digital Elevation Models
•  DEM Viewing Software
•  Quick Guide to Making 3-D Views from DEMs

Digital Elevation Models

•  What is a Digital Elevation Model?
•  Three Dimensional Viewing
•  DEMs and GIS Intergration
•  DEM Related Links

What is a Digital Elevation Model (DEM)?
Digital elevation models (DEMs) are collections of elevation points for an area. Among other things, certain software can display DEMs to show the terrain surface in three dimensions and from a choice of viewpoints. DEMs are created by collecting elevations and referencing them to corresponding points in the mapped area. The elevations add a Z value to the ground's X and Y horizontal coordinates.

A systematic grid pattern or a stratified method of data collection is used to gather elevation coordinates. These two methods are often referred to as regular and irregular grids respectively. A regular grid has points equally spaced regardless of the shape of the terrain. An irregular grid samples extra points or a larger percentage of points in areas of increased relief variation, such as along ridges, mountains, and valleys.

The spacing and density of the elevations points directly effects the accuracy of a DEM. The higher the density of elevation points recorded results in the model's depiction of the actual terrain to be more accurate. Of course, this means more points have to be recorded which, unfortunately, requires more work and more demand for data storage space, and consequently also leads to higher costs. Elevations may be surveyed or collected using traditional surveying methods or the Global Positioning System (GPS). However, in most cases elevations are calculated from contour lines on pre-existing maps. Typically, the contour lines on these maps are created from aerial photographs using photogrammetry. In recent years it has become common to create DEMs directly from the photographs, and later derive contours from the DEM if desired.

A major issue in building a DEM is in selecting points. Generally, the more known elevations the better, but depending upon the variability of the landscape, there is a point where the benefits of more points no longer become clear. Over flat areas, only a handful of points may be necessary, but over irregular terrain, a very dense matrix of points may be needed to adequately capture the variations in the land surface.

Computer driven mathematical algorithms can be used to interpolate extra points if they are desired. Interpolation algorithms use existing points to estimate the elevations of points that lie between known elevation points. This is basically how a computer displays a DEM. 3-D modeling software use interpolation algorithms to create a dense grid of points, which gives the appearance of a continuous three-dimensional surface.

Click on the thumbnail image of the USGS topoquad of Holy Hill to see how a 3D digital elevation model compares to a 2D topographic map.

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Three Dimensional Viewing
Prior to computers, elevation models could only be compiled as lists or tables, but not easily displayed or analyzed. With this in mind, the importance of computer software in DEM analysis can be realized. In order for the data to be fully useful, software must not only display the model, but it must allow for calculating slopes, volumes, and other three dimensional qualities.
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DEMs and GIS Intergration
High end GIS software can integrate DEMs into spatial analysis. Land erosion susceptibility, viewshed analysis, flooding prediction, mining potential, site selection, and countless other projects are greatly enhanced by 3-D modeling. Other layers of information such as land cover and hydrography can be overlaid onto DEMs for a 3-D view of the landscape. Satellite imagery or digital orthophotos can also be overlaid onto DEMs.

This perspective view of La Crosse was made by overlaying a 1-meter resolution Digital Orthophoto Quarter-Quadrangle on a corresponding DEM, with the same projection. The DEM elevation has been vertically exaggerated by a factor of 4.

This is a reflectance image made from the USGS 1:24,000-scale DEM of the Wisconsin Dells North Quad. This portrayal is vertically exaggerated by a factor of two and is overlaid with the USGS 1:24,000-scale hydrography Digital Line Graph (DLG).

This view of Swan Lake State Wildlife Area, near Portage, WI was made by overlaying the Portage 1:24,000-scale DRG on the corresponding DEM. The DEM elevation has been vertically exaggerated by a factor of 2.

DEM Related Links
DEMs are a form of Digital Terrain Mode (DTM). DTM is a general term for digitally created three-dimensional representations of the landscape. Another form of a DTM is a Triangulated Irregular Network (TIN). TINs are similar to DEMs, but they contain points in an irregular pattern that follows the terrain. The trigonometry involved is far too complex to summarize here, but if you want to learn more about TINs or DTMs in general, check out the links below.

•  Modeling the third dimension
•  The importance and need for DEMs
•  Terrain Analysis
•  DEM Mapping Standards
•  College Chapter on DEMs