Why does the coverage of NED 1/3 arc second data appear to include a greater area than that covered by the 10 meter DEMs?
A. The extent of 1/3-arc-second NED appears greater because some areas are derived from data other than the 10-meter DEMs. The USGS 10-m DEMs are the primary source data for production of 1/3-arc-second NED. Additionally, non-USGS elevation data, derived from lidar and photogrammetric data collections, are used as source data for 1/3-arc-second NED. NED is processed in 1-degree by 1-degree tiles, and when a tile is not fully covered by 10-meter DEMs and/or other high-resolution source data, 30-meter DEMs are oversampled to fill out the tile. The NED spatial metadata delivered with each order can be queried to determine the source data used to produce 1/3-arc-second NED over any given area. As new high-resolution source data become available, either 10-meter DEMs or other sources, the data derived from 30-m DEMs will be replaced.
