What horizontal datum, vertical datum, and geoid model are used for the NED?
The horizontal datum for the NED is NAD 83. The NAD 83 datum uses the GRS 80 ellipsoid (as an aside, the GRS 80 ellipsoid is nearly identical to the WGS 84 ellipsoid, but not exactly). The vertical datum for the NED is NAVD 88. NAVD 88 is an orthometric datum, meaning that it is based on a representation of mean sea level. Most of the source data used to produce NED is referenced to NGVD 29, an older orthometric datum for the U.S. which has now been superceded by NAVD 88. Thus, during NED production there is a vertical datum transformation performed to convert the source elevation data from NGVD 29 to NAVD 88. The National Geodetic Survey (NGS) tool VERTCON is used to perform the vertical datum transformation. Note that NED production does not include any use of a geoid model. Most elevation data users want the data in reference to the orthometric (sea level) datum (NAVD 88). However, if an application requires the elevations to be in reference to the ellipsoid, the orthometric-referen
