National Elevation Dataset
http://gisdata.usgs.net/ned/
To represent terrain, we were able to drawn on the new USGS National Elevation Dataset (NED). This seamless one arc-sec (approx 30m) digital elevation data represents a significant enhancement to the previous USGS 7.5-min DEM products.
The typical 7.5-min DEM shown on the left exhibits a vertical “seamline” in the center where two 7.5-minute Quad sheets join as well as much higher-frequency horizontal “cornrows” – an artifact of the automated stereo correlation process used to generate the DEM from stereoscopic aerial photography.The NED example shown on the right has extensively processed to minimize such artifacts and to enhancetopographic expression by use of a synthetic drainage model. You can download up to 10mb or less for free from the website http://gisdata.usgs.net/seamless