Commonly Asked Questions

How were these county DEMs produced?

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

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.