Jon Larimer wrote a program to convert USGS Digital Elevation Model map files to TGA format.
The USGS maintains DEM files which have data for the United States in 1-degree, 30-minute, 15-minute, and 7.5-minute formats. The 30-minute DEMs are available for pretty much the whole USA.
I have subsequently modified the program a little bit to produce higher resolution TGA files suitable for use as heightfields in POV-Ray (the raytracer), for instance.
You can download the ZIP file with DOS executable and source, or just the source file, if you prefer.
Here are some examples of heightfield renderings comparing the normal operation (256 levels) and the modified operation (65536 levels):
The original:
Modified:
