NED (National Elevation Dataset)
The National Elevation Dataset (NED) is a raster product assembled by the US Geological Survey (USGS) from several other US data sets. It is designed to provide elevation data for the United States in a seamless form with a consistent datum, elevation unit, and projection. The NED has a horizontal resolution of 1 arcsecond (approximately 30 meters at the equator) for the conterminous US, Hawaii, and Puerto Rico, and 2 arcseconds for Alaska. Note, however, that actual pixel dimensions (in meters) vary with the cosine of latitude. Since most of Alaska is north of 60 degrees latitude, and since cos(60)=1/2, pixel dimensions in Alaska are also close to 30 meters. The NED will be updated to incorporate data of higher resolution or quality as it becomes available.
Each NED tile spans a 1 degree longitude by 1 degree latitude region, and their filenames are constructed from the latitude and longitude of their upper left (northwest) corner. For example, "n40w100" indicates that the northwest corner is at 40 degrees north latitude and 110 degrees west longitude.
During the NED assembly process, source data was corrected when necessary to minimize artifacts and to provide better edge matching near seams between adjacent source DEMs. The source data included USGS 7.5-minute DEMs (UTM, 30 meter pixels) for which edge matching problems are not uncommon.
How to Import
Obtain NED DEMs in the ARC FLT format. Choose Import DEM → ARC FLT Binary Raster from the File menu and then select the file, which should have the filename extension "*.flt". Before using tools in the Extract menu, go to the Planet Info panel of the File → Set Preferences dialog and set the Earth ellipsoid to either "GRS 1980" or "WGS 1984" (these are almost identical). Otherwise calculations of lengths, areas, slopes, etc. will be less accurate.Data Source
US Geological Survey (USGS) of the US Department of the Interior
NED DEMs became available in 2001 for download from the Internet and on standard distribution media. An Internet browser tool called National Map Viewer is provided as an aid to ordering subsets of NED that span a user-specified range of latitudes and longitudes.
NED Home: http://ned.usgs.gov
NED FAQ: http://ned.usgs.gov/faq.asp
NED Downloads via National Map Viewer: http://viewer.nationalmap.gov/viewer/
Historic DEMs: http://ned.usgs.gov/Ned/historic.asp
USGS Home: http://www.usgs.gov
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