Commonly Asked Questions
How were these county DEMs produced?
The following procedure was used to produce a county or city DEM:
- Merged DEMs were downloaded from the USGS Seemless Web site at
http://seemless.usgs.gov. This site
provides a seemless continental elevation data set for the United States. Once
downloads were completed, ArcGIS 8.3 was used to merge the DEMs into a single
data set that encompassed the state of Virginia
- Using a 1:24,000 Virginia county boundary ArcInfo coverage, a custom AML
(Arc Macro Language) script was written to derive county elevation data sets.
The algorithm consisted principally of selecting a county or city polygon,
converting it to a raster data set, and then creating a subset from the large
statewide DEM. The result was then exported as an ASCII raster format and then
compressed for faster downloading via the Internet. This AML took about five hours to execute on a 1.4 gigahertz
What is the resolution of the DEMs?
The county DEMs are created from USGS 7.5' DEMs. Each elevation cell
represents either 10 meters or 30 meters, depending on the availability of the
newer USGS 10 meter product. Elevation units are in meters. Any USGS DEMs
that originally had elevations in feet have been converted to meters.
How often will Radford University update these files?
Since the USGS continually updates its DEM products, we will reproduce the
data sets in their entirety annually. Look for new files next November. Please
note that many county data sets will remain the same, since the USGS only
updates some files each year.