Commonly Asked Questions

How were these county DEMs produced?

The following procedure was used to produce a county or city DEM:

• Using ArcInfo 8.0.2, DEMs were imported as grids. The PROJECTCOMPARE command was used with the FULL option to ensure that vertical distance units are converted to X and Y units. This means that those DEMs that measure distance in meters and elevation in feet now use meters for elevation
• The county boundary was converted to a grid in ArcView
• Using the GRID module in ArcInfo 8.0.2, the new county boundary grid was specified as a mask. Future Grid processing commands are limited to the area encompasses by the county grid, making it a "cookie cutter" 
• The MERGE command in Grid was used to combine all converted DEMs into a single grid. The grid conforms to the shape of the county because of the SETMASK command used in the step above.
• The LATTICEDEM command was used to convert the grid back to USGS DEM format.
• WinZip was used to compress the county DEM in order to make downloads from the website speedier.

What projection do the county DEMs use?

The county DEMs all use the UTM coordinate system. The western part of Virginia falls in zone 17 and the eastern in zone 18.

What is the resolution of the DEMs?

The county DEMs are created from USGS 7.5' DEMs. Each elevation cell represents 30 meters. Distance units and elevation are in meters. Any USGS DEMs that originally had elevations in feet have been converted to meters.

Virginia has independent cities as well as counties. How are they handled?

If a city is completely enclosed by a county, say like the cities of Salem and Roanoke are encompassed by Roanoke county, the city is included twice. It will appear as an independent DEM that you may download. The city is also included in the surrounding county's DEM, too. In this example, you could download separate DEMs for the cities of Salem and Roanoke as well as a DEM for Roanoke county that would include the two cities.

If a city lays on the boundary between two counties, like Radford City lies between Pulaski and Montgomery counties, it is included just once, as a separate DEM for you to download.

Why does a solid vertical line sometimes seem to run through part of my county dataset?

The most likely reason for this is that there is a gap between the two adjacent 7.5' DEMs that make up this part of the county. ArcInfo tries to interpolate elevation values as it merges the data sets but this is not always successful if the gap between DEMs is too large.

What counties are available?

The web page indicates which counties are currently available. Zone 17 is now complete. Zone 18 counties and counties that fall in both zones will become available this fall.