The USGS Medium Scale Shaded Relief from The National Map is a cached tile service that contains shaded relief elevation layers derived from the National Elevation Dataset (NED). The NED was developed by merging the highest resolution, best-quality elevation data available across the United States into a seamless raster format. Medium scale shaded relief represents NED data with a horizontal grid spacing of 1 arc-second (approximately 30 meters) as if it were a surface illuminated by a light source 45 degrees above the northwest horizon. Medium scale shaded relief can be viewed from 1:4.6M to 1:578,000 in The National Map viewer. Go to http://ned.usgs.gov/ and http://extract.cr.usgs.gov/distmeta/servlet/gov.usgs.edc.MetaBuilder?TYPE=HTML&DATASET=NED for more information on 1 arc-second NED.