HOWTO import USGS elevation data
The US Geological Survey offers a variety of digital elevation models through the Seamless Distribution System. The highest resolution offered is 1/9 second (= about 3 meters) with limited coverage of the USA. Most areas not covered by 1/9 arc-second are covered in 1/3 arc-second, with roughly 10-meter resolution. Here is an approach to working with them in GRASS.
Importing USGS 1/3 arc second NED datasets into GRASS
1. Download the datasets you desire through the provided web interface. They will arrive in a zip archive of a directory containing a subdirectory containing a file called w001001.adf, which is the main data file (tip: this is the biggest file).
2. Create a new GRASS location to work with the DEMs.
Pick one DEM to start with. From a GRASS shell:
cd 18437824/18437824(example directory names --- replace with your actual names)r.in.gdal location=name_of_your_new_location in=w001001.adf out=dem
3. Start GRASS (or restart) in the new location.
grass name_of_your_new_location/PERMANENT
4. If you have more than one DEM archive, import additional files as needed (note the -e flag, which causes the location's bounds to be extended to accomodate the new data):
r.in.gdal -e in=w001001.adf out=dem2- .. etc.
5. Use r.patch to combine the tiles into one continuous map, if desired.
Alternatives to the USGS DEM data
Other sources of digital elevation models include: