The National Map
Data on America’s elevation have typically been represented as contour lines and bare earth digital elevation models (DEMs) in The National Map. With the 3D Elevation Program (3DEP) operational, light detection and ranging (lidar) and interferometric synthetic aperture radar (ifsar) data (in Alaska) are now the primary sources of elevation data for 3DEP, and provide opportunities for measuring, mapping and monitoring not only the bare earth surface, but above ground features as well. As a result, in addition to the traditional bare earth DEMs, USGS now provides the source lidar point clouds, ifsar digital surface models (DSMs) and orthorectified radar intensity images (ORIs) over every area where data have been acquired for 3DEP and The National Map. Contours derived from these elevation data are used in the US Topo digital topographic maps.
Three bare earth DEM layers in 3DEP are nationally seamless and are distributed in geographic coordinates at 1/3, 1, and 2 arc-seconds. Two high resolution layers over the conterminous U.S., 1/9 arc-second and 1-meter, are seamless within projects but not across projects. The 1-meter bare earth DEM dataset will be populated as new data are acquired in 2015 and beyond. Alaska is partially covered by a 5-meter DEM that will eventually become the highest-resolution elevation dataset over the entire state. Each of the seamless bare earth DEM layers are derived from the highest quality DEMs available for any geographic location within the conterminous United States, Alaska, Hawaii, Puerto Rico, U.S. territorial islands, Mexico and Canada. The extent of geographic coverage varies by layer.
For over 15 years, the elevation layer of The National Map was the National Elevation Dataset (NED). The NED provided basic bare earth elevation information for earth science studies and mapping applications in the United States. With 3DEP operational, the seamless bare earth DEMs that were called "NED" have been renamed and are considered just one component of elevation in The National Map.