DEFINITION OF DIGITAL ELEVATION MODELS

A digital elevation model (DEM)

is a digital file made up of topographic elevations for known

ground positions at uniform horizontal intervals. The U.S.

Geological Survey is responsible for five separate digital elevation

models. The five models are all kept in the same matter

of data, but the intervals, geographic references, coverage

areas, and accuracies are different. The models include the

following:

1. 7.5 Minute DEM 30 × 30 meter data intervals

2. 1-Degree DEM 3 × 3 arc second data intervals

3. 2-Arc-Second DEM 2 × 2 arc second data intervals

4. 15-Minute Alaska DEM 2 × 3 arc second data intervals

5. 7.5-Minute Alaska DEM 1 × 2 arc second data intervals

The DEMs are used for three-dimensional graphics that

display certain terrain characteristics. They provide scientists

with a computer application that allows them to view the terrain

in great detail. DEMs have also been used for many nongraphic

applications such as the administration and

exploration of natural energy resources. The U.S. Geological

Survey is currently planning to convert all DEMs to Spatial

Data Transfer Standard (SDTS), which is a widely used universal

system used to transfer data between separate and different

computer operating systems.

disconformity See UNCONFORMITY.

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