The National Elevation Dataset (NED) is a new raster product assembled by the U.S. Geological Survey (USGS). The 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, permit edge matching, and fill sliver areas of missing data.
The NED has a resolution of 1 arc-second (approximately 30 meters) for the conterminous United States, Hawaii, and Puerto Rico and a resolution of 2 arc-seconds for Alaska. National Elevation Dataset data sources have a variety of elevation units, horizontal datum, and map projections. In the NED assembly process, the elevation values are converted to decimal meters as a consistent unit of measure, North American Datum 1983 is consistently used as horizontal datum, and all the data are recast in a geographic projection. Older digital elevation models (DEM) 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.
As higher resolution or higher quality data become available, the NED is updated to incorporate the best available coverage. As the 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 incorporate these sources. 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. These original elevation files are currently available at http://seamless.usgs.gov/. In cases where 7.5-minute DEM's have 10-meter resolution, the original source data will be at a higher resolution than the NED. As more data become available at a finer resolution than that of the NED, the feasibility of developing a finer resolution NED will be investigated. Complete documentation is available at http://ned.usgs.gov//.
National Elevation Data Sources and Documentation
The Federal Geographic Data Committee's content standard for digital geospatial metadata (http://www.fgdc.gov/metadata/geospatial-metadata-standards) will be used to document NED data. Metadata about the individual source datasets used to assemble NED are presented in a spatially referenced form. The polygonal footprint of each part used from a source dataset is retained during NED assembly to provide the spatial context. The attributes of each source dataset, such as original resolution, production method, and date entered into NED, are linked to this polygonal footprint. All of these source polygons together form a national coverage. Through this spatially referenced metadata, the information is made available to the user regarding the source data for any area of NED. For example, a NED user might use the spatially referenced metadata to identify the parts of a study area that were produced by obsolete production methods.
Current Phase One Product Specifications
NED is provided in a geographic projection with NAD83 horizontal datum. The cell size is
one arc-second (approximately 30 meters). NED elevation values are in decimal meters. If
Arc Grid or GRIDFLOAT formats are selected, the elevation value units will remain floating
point meters. If the bil format is selected, which is integer only, the elevation values
are converted from floating point to 32-bit integer. The customer has the choice of
decimeter or centimeter units. The bil format is provided to meet the needs of those
customers who prefer integer data. One reason customers may prefer integer is that they
have software packages that do not
function on floating point data. As more information on the NED becomes available,
it will be accessible through the Internet at http://ned.usgs.gov//.
Data is currently available only on CD. Samples are available at this site to
download and view.
For additional information regarding this Web page, contact Deb Southworth Green , in the Division of Information Resources and Technology Management, at Deb_Green@fws.gov
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Keywords=GIS, data, DEM, elevation, NED
Last Modified
May24, 2007