Terrain analysis involves the processing and graphic simulation of elevation data. Terrain analysis software functions usually work with topographic data (also called terrain data or elevation data), in which an elevation (or Z value) is recorded at each X,Y location. However, terrain analysis functions are not restricted to topographic data. Any series of values, such as population densities, ground water pressure values, magnetic and gravity measurements, and chemical concentrations, may be used.
Topographic data are essential for studies of trafficability, route design, nonpoint source pollution, intervisibility, siting of recreation areas, and so forth. (Welch, 1990). Especially useful are products derived from topographic data. These include:
- slope images—illustrates changes in elevation over distance. Slope images are usually color-coded according to the steepness of the terrain at each pixel.
- aspect images—illustrates the prevailing direction that the slope faces at each pixel.
- shaded relief images—illustrates variations in terrain by differentiating areas that would be illuminated or shadowed by a light source simulating the sun.
Topographic data and its derivative products have many applications, including:
- calculating the shortest and most navigable path over a mountain range for constructing a road or routing a transmission line
- determining rates of snow melt based on variations in sun shadow, which is influenced by slope, aspect, and elevation
Terrain data are often used as a component in complex GIS modeling or classification routines. They can, for example, be a key to identifying wildlife habitats that are associated with specific elevations. Slope and aspect images are often an important factor in assessing the suitability of a site for a proposed use. Terrain data can also be used for vegetation classification based on species that are terrain-sensitive (for example, Alpine vegetation).
Although this chapter mainly discusses the use of topographic data, the ERDAS IMAGINE terrain analysis functions can be used on data types other than topographic data.
See Geographic Information Systems for more information about GIS modeling.