Posted By Mister Geodesist on 4/24/2008 at 2:46 PM
The notion was raised this morning that NAD83 is somehow not a three dimensional (3D) coordinate system. As someone who was involved in the development of NAD83 from 1974 on until after the start of the HARNS and the beginning of the CORS I can assure everyone that NAD83 has been a 3D system since its beginning.
Included in the initial NAD83 to control it nationwide was a national network of satellite Doppler stations as well as a number of VLBI stations. These stations had 3D coordinates in the original NAD83 adjustment.
The bulk of the original stations were conventional stations and provided only latitude and longitude. But almost as soon as NAD83 was completed GPS came on line and there was a need to add 3D GPS stations. The only NAD83 stations with sufficently accurate 3D NAD83 coordinates at the beginning were the VLBI stations. And there were no accurate orbits. So GPS stations that were co located with VLBI stations were held fixed and the new GPS stations adjusted to them.
I drew up the first list of standard NAD83 coordinates to be used at VLBI sites in performing transformations to NAD83.
As the IERS performed more and more ITRF adjustments it was decided that some standard way should be adopted to go from ITRF to NAD83. NGS, together with Geodetic Survey of Canada, developed a standard 14 parameter transformation. It was decided that to assure no NAD83 coordinate system incompatibility as the IERS changed systems the transformations would be made backwards compatible.
The CORS coordinates are labeled NAD83 (1996) because ITRF96 was the basis for the initial transformation.
NAD83 has always been a 3D coordinate system. And NGS has made a special effort to see that, except for a small scale change not long after NAD83 was established this ooordinate system has not changed.