NED (National Elevation Dataset)DescriptionThe National Elevation Dataset (NED) is a new 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. 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. Elevation units are decimal meters. North American Datum 1983 (GRS 80 ellipsoid) is used as the horizontal datum, and the data is in the Geographic "projection". The NED is nearing completion and will be distributed in the Spatial Data Transfer Standard (SDTS) Raster Profile format. The NED will be updated to incorporate data of higher resolution or quality as it becomes available. 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. Data SourceUS Geological Survey (USGS) of the US Department of the Interior Data AvailabilityNED DEMs will be available in 2001 for download from the Internet and on standard distribution media. An Internet browse tool will be provided as an aid to ordering subsets of NED that span a user-specified range of latitudes and longitudes. Web SitesUSGS Home: http://www.usgs.gov/ NED Home: http://ned.usgs.gov NED Fact Sheet: http://ned.usgs.gov/about.asp NED Fact Sheet: http://ned.usgs.gov/faq.asp Copyright (c) 1998-2008, Rivix, LLC. All rights reserved. |