The final data set includes DEMs from the US Geological
Survey (USGS) National Elevation Database (NED)
and the Shuttle Radar Topography Mission (SRTM). Both
data sets are available online at http://edc.usgs.gov/products/
elevation.html. The NED DEM is based on contour data
supplemented with photogrammetric data acquired in
1970–1980s, has a horizontal resolution of about 60 m,
and a root-mean-square (RMS) vertical error of 15 m
(Gesch, 1994). The NED DEM has a horizontal datum of
the North American Datum 1927 (NAD 27) and vertical
datum of North American Vertical Datum 1988 (NAVD 88).
The 1-arc-second (about 30 m) SRTM DEM, acquired in
February 2000, has a relative vertical accuracy better than
10 m and an absolute vertical accuracy better than 16 m
(Farr & Kobrick, 2000; Shuttle Radar Topography Mission
(SRTM) Spatial Metadata Dataset at http://www.metrokc.
gov/gis/sdc/radster/elevation/ShuttleRadarTopographyMission(
SRTM)SpatialMetadata.html). The SRTM DEM has a
horizontal datum of the World Geodetic System 1984 (WGS
84) and a vertical datum of NAVD 88. Before differencing,
the NED DEM was over-sampled to match the horizontal
datum and pixel size (30 m) of the SRTM data.