Background

The United States Geological Survey (USGS) produces digitally-encoded terrain data in the form of regular grids called "digital elevation models", or DEM files. A DEM file contains elevations for ground positions at regularly spaced horizontal intervals.

At the USGS, DEM's have been used in combination with digital raster graphics (DRG's), digital line graphs (DLG's), and digital orthophoto quadrangles (DOQ's) to both enhance the visual information for data extraction and revision purposes and to create aesthetically pleasing and dramatic hybrid digital images. Non-graphic applications such as modeling terrain and gravity data for use in the search for energy resources, calculating the volume of proposed reservoirs, and determining landslide probability have also been developed.

Terrain Tools will read both DEM and SDTS formats. 

Once in Terrain the DEM data can be used for visualization, contouring, profiles, volumes etc.

Importing DEM data

1) Select menu File | Insert File, and choose USGS in the Files of Type dropdown and enter the name of the file.

2) The Import Options dialog box shown above will appear.  The Link Terrain check box will automatically create a TIN model from the elevation points.  The Spacing Increment allows you to thin the incoming points.  A spacing increment of 10 will accept every 10th point.  

NOTE: DEM's can be very large.  It is advisable that the first viewing of a DEM be done with a spacing increment of 10.

3) Depending on your current projection, you may be prompted to convert coordinate systems.

4) Once the DEM has been imported it can be used for surface modeling and display.

USGS DEM - from the San Francisco Bay Area

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