In recent years, contour values have been digitized allowing them to be manipulated into versatile displays of topographic data as digital elevation models or DEMs. Digital terrain models (DTMs) are variants that show additional landscape attributes. We can use existing maps as inputs by tracing contour lines on a digitizing tablet or table. The data are organized into cell arrays whose X-Y positions in the rectangular grid are related to map coordinates. Each cell has a single value representing the average elevation of the land surface within it. The finest-sized cells are 30 meters on a side, associated with the 7.5 minute topographic quadrangles mapped by the USGS using the Universal Transverse Mercator (UTM) coordinate system. Only a fraction of the maps at this scale have been digitized, as yet. This digitizing is also true for 15 and 30 minute maps. To date, all of the 50 U.S. states, except parts of Alaska, mapped at 1:250,000 scale (extending over 1 degree by 1 degree in eastern states and 1 degree by 2 degrees in western states) have now been digitized at a cell size of three arc seconds. The data are stored in east-west profiles.
These DEM data sets have important applications. They allow for rapid reconstruction of contour maps and for plotting elevation profiles. Also, we can easily do various kinds of photogrammetric calculations with the numbers. Image processing systems can edit and filter these data to enhance the display products and merge data sets into larger maps. Cartographers can present surfaces as X-Y-Z plots, where the Z dimension appears as regularly spaced vertical lines whose lengths are proportional to elevation. These plots are available in several formats:
Shaded relief maps, which assigns different shades of gray to slopes, depending on elevation, or a variant, which draws in shadowing to selected slopes depending on their orientation (aspect) and on sun direction and azimuth. By shifting these parameters, we create different renditions that bring to light different surface features and trends.
Color density slices, which color-code elevations.
Perspective Views. Since one can readily calculate geometric variables, such as orientation and height from the digitized data, the surfaces can be recreated to look like oblique photos. Then it is easy to examine such surfaces from different perspectives by rotating the view horizontally (along a vertical axis) or changing the look angle.
Draped Views. The ultimate display is usually one which registers or "drapes" a surface image, such as a Landsat scene, onto the DEM array (data cells match with pixels), causing that surface and the features on it to appear in some form of 3-D display (e.g., in a perspective view or by creating pseudo-stereo pairs as a stereo model). Likewise various thematic maps, as for example, land use, urban structure, or geology, can be put into 3-D mode.
Examples of these DEM-based products abound on the Internet. We show just a few that are typical:
Check first this gray level relief map, constructed from 1:250,000 DEM data, of the Susanville area in the Sierra Nevada Mountains:
Next, examine the variant, in which shadow effects emphasize relief, by switching to a grand scale view of the entire conterminous U.S. Locate your home area.
An excellent example of a map in which the elevations are color-coded is this DEM version showing the state of Wyoming. The data source is a series of digitized 30' topographic maps. Lower areas are in green, and higher are in yellows, then brown.
Let's now peer at a spectacular scene, looking at the Grand Canyon, as shown in a perspective view developed from DEM data and artificially colorized to resemble the rock units. But first, to gain familiarity with this incredible "ditch," carved over millions of years by the Colorado River, examine this Landsat nearly full 1972 scene. The snow-covered area is the Kaibab Plateau, some 2000 feet higher than the South Rim.
Next, a later view of the west half taken (November 1,1993) by the Japanese JERS-1:
Now let's look at the DEM rendition: