Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Digital Elevation Model Files

A digital elevation model (DEM) data file is a digital representation of elevations, using a raster format. A DEM consists of an array of elevations for many ground positions at regularly spaced intervals. In this dataset they appear as square cells, or, visualizing them three dimensionally, “square posts” whose flat tops represent the height of the land or water at the particular geographic positions.

____ 39. Display a layer from the Digital Elevation Model (DEM) that is in the form of an ArcInfo Raster:27 From [___]IGIS-Arc\River add a layer based on the Coletown Quadrangle digital elevation model named COLE_DEM. What is the range of elevations shown in the T/C? _____ feet to ______ feet. Drag its title to the bottom of the T/C. Turn off the two image layers above it. Zoom to the extent of the DEM layer. You can get an idea of the topography by looking at the DEM. White areas are the highest elevations; dark areas the lowest. See Figure 2-31. Select the Identify tool on the Tools toolbar. Click a light area that doesn’t have other features (such as vectors) nearby. The Identify window will show the elevation in two places: under cole_dem and as the Pixel value. (You will also see a stretched value number, which should be close to 255. This indicates the intensity of the “color” at that location. Pure white would be 255 which corresponds to a byte with all bits set to 1s, that is, 2 to the 8th power.) Set the “Identify from” text box to COLE_DEM ...