Terrain data are usually expressed as a series of points with X,Y, and Z values. When terrain data are collected in the field, they are surveyed at a series of points including the extreme high and low points of the terrain along features of interest that define the topography such as streams and ridge lines, and at various points in between.
DEM and DTED are expressed as regularly spaced points. To create DEM and DTED files, a regular grid is overlaid on the topographic contours. Elevations are read at each grid intersection point, as shown in the following figure.
Regularly Spaced Terrain Data Points
Elevation data are derived from ground surveys and through manual photogrammetric methods. Elevation points can also be generated through digital orthographic methods.
To make topographic data usable in ERDAS IMAGINE, they must be represented as a surface, or DEM. A DEM is a one-band image file where the value of each pixel is a specific elevation value. A gray scale is used to differentiate variations in terrain.
DEMs can be edited using Raster Editing capabilities in ERDAS IMAGINE. See Raster Data for more information.