USGS offers digital elevation maps as rasters [1]. There is also the 10-meter SRTM data from shuttle SAR [2]. Combining elevation with the USGS topo maps visually for 3D is an interesting image processing task.

What hardware are you using in your tracker?

[1] http://nationalmap.gov/elevation.html

[2] https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Missi...