National Elevation Dataset
The National Elevation Dataset (NED) is a raster product assembled by the U.S. Geological Survey. The NED is designed to provide National elevation data in a seamless form with consistent datums, elevation unit, and coordinate reference system. The NED assembly process involves edge matching and mosaicking elevation data into NED layers. Seamless NED layers are available at the following horizontal resolutions: 1/ 3 arc-second (approximately 10 meters) resolution for the conterminous United States and portions of Alaska, 1 arc-second (approximately 30 meters) for the conterminous United States, Hawaii, Puerto Rico, U.S. territorial islands, Mexico, and Canada, and 2 arc-second resolution (approximately 60 meters) over all of Alaska. Coverage of Alaska at 1/3 and 1 arc-second will increase significantly through 2016 as 5-meter IFSAR source data are acquired under the Alaska Statewide Digital Mapping Initiative.
In addition to seamless elevation layers, NED data are also available at a 1/9 arc-second (approximately 3 meters) resolution. At this time the 1/9 arc-second NED layer covers approximately 1/3 of the United States. These data are seamless within individual acquisition projects (variable geographic extents), but, are not not seamless across projects; that is, no blending, edge matching or mosaicking is performed between projects. Individual 1/9 arc-second projects are delivered to the original project boundary; this differs from the seamless layers, where data are delivered to the full extent of the tile (or the extent of available source data). The area of 1/9 arc-second coverage will increase as new lidar projects are acquired under the 3D Elevation Program (3DEP).
The NED layers are derived from a variety of data sources that are processed to a common coordinate system and unit of vertical measure. NED data are distributed in digital elevation model (DEM) format with decimal-degree geographic coordinates referenced to the North American Datum of 1983 (NAD83). All elevation values are in meters and, over the conterminous United States, are referenced to the North American Vertical Datum of 1988 (NAVD 88). The vertical reference will vary in other areas.
Older source DEM's produced by methods that are now obsolete have been filtered during the NED assembly process to minimize artifacts that are commonly found in data produced by these methods. Artifact removal greatly improves the quality of the slope, shaded-relief, and synthetic drainage information that can be derived from the elevation data. The artifact removal filtering process does not eliminate all of the artifacts. In areas where the only available DEM is produced by older methods, then "striping" may still occur. Processing the NED also includes steps to adjust values where adjacent DEM's do not match well, and to fill sliver areas of missing data between DEM's. These processing steps ensure that the NED has no void areas and minimal artificial discontinuities.
All NED products are distributed through The National Map. On April 1, 2014 the USGS began distributing newly acquired digital elevation models (DEMs) in their original projection and resolution. These products are available for download through The National Map Viewer.