Many source DEM's required datum and elevation conversions to the
specifications
of the NED database.
The NED development process incorporated
National Geodetic Survey and U.S. Army Topographic Engineering Center (TEC)
software
to enable the necessary horizontal and vertical datum conversions.
The predominate horizontal datum conversion was from North American Datum of
1927 (NAD 27) to North American Datum of 1983 (NAD 83). The predominate
vertical datum conversion was from National Geodetic Vertical Datum of 1929
(NGVD 29) to North American Vertical Datum of 1988 (NAVD 88). Other required
horizontal datum conversions were World Geodetic System 1972 Datum (WGS 72)
to World Geodetic System 1984 (WGS 84), and conversion of Old Hawaiian Datum
(OHD) and Puerto Rico Datum of 1940 (PRD) to NAD 83. Other vertical datum
conversions included conversion of local mean sea level (LMSL) to NAVD 88.
Alaska has a horizontal datum of NAD27 and a vertical datum of NAVD29.
DEM files were also found with elevations stored in either meters or feet.
Dataset development required conversion of all elevations to a common unit.
Conversion of elevations to decimal meters was adopted in order to preserve
the higher precision of data already in feet. These datum and elevation
conversions were performed as each DEM file was transformed into a
geographic projection. (courtesy of the USGS) |
