NATIONAL ELEVATION DATASET
The National Elevation Dataset (NED) is a new raster product assembled by the U.S. Geological Survey. NED is designed to provide National elevation data in a seamless form with a consistent datum, elevation unit, and projection. Data corrections were made in the NED assembly process to minimize artifacts, perform edge matching, and fill sliver areas of missing data. NED has a resolution of one arc-second (approximately 30 meters) for the conterminous United States, Hawaii, Puerto Rico and the island territories and a resolution of two arc-seconds for Alaska. NED data sources have a variety of elevation units, horizontal datums, and map projections. In the NED assembly process the elevation values are converted to decimal meters as a consistent unit of measure, NAD83 is consistently used as horizontal datum, and all the data are recast in a geographic projection. Older 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. Figure 2 illustrates the results of this artifact removal filtering. NED processing 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 NED has no void areas and artificial discontinuities have been minimized. 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.
Figure 2. A shaded-relief representation of the Rockypoint, Wyoming 7.5-minute DEM is shown above on the left. The same area is shown on the right after NED artifact filtering has been performed. The super-imposed red lines are synthetic drainage networks derived from each elevation dataset.
As higher-resolution or higher-quality data become available, the NED is updated
bi-monthly to incorporate best-available coverage. As USGS's 7.5-minute and
15-minute digital elevation products near completion for the conterminous United
States and Alaska respectively, NED data will soon be derived from these sources
at a minimum. For the small areas that are not yet covered, the lower-resolution
30-minute and 1-degree USGS DEM products were interpolated to obtain values
used in NED. The NED Data Dictionary, release notes, and update information
can be found at http://ned.usgs.gov/downloads.asp
A visual index of the properties of the individual source datasets and update
information is at the Data
Source Index for NED. The source 7.5-minute DEM's can have a wide range
of source dates.