Modern terrestrial reference systems part 2: The evolution of NAD 83

he first realization of NAD 83 was introduced in 1986 by a group of institutions re p resenting the various North American countries to upgrade the previous reference system; that is, the North American Datum of 1927 or NAD 27. In particular, the National Geodetic Survey (NGS) represented the United States, and this federal institution officially refers to the first NAD 83 realization as NAD 83 (1986). For this realization, the group of institutions relied heavily on Doppler satellite observations collected at a few hundred sites to estimate the location of the Earth's center of mass and the orientation of the 3D cartesian axes. They also relied on these same Doppler observations to provide scale for NAD 83 (1986). More precisely, the group of institutions relied on 3D Doppler-derived positions that had been transformed by: • a translation of 4.5 m along the z-axis • a clockwise rotation of 0.814 arc sec onds about the z-axis • a scale change of-0.6 ppm The Doppler-derived positions were so transformed to make them more consistent with the very long baseline inter-ferometry (VLBI), satellite laser ranging (SLR), and terrestrial azimuth measurements that were available in the early 1980s. While NAD 83 (1986) is 3D in scope, NGS adopted only horizontal coordinates (latitude and longitude) for over 99% of the approximately 250,000 U.S. control points that were involved in defining this reference frame. Unfortunately , this first realization of NAD 83 occurred a few years before GPS technology made the vertical dimension economically accessible. GPS Changed Everything A round the same time that NGS adopted NAD 83 (1986), the agency had begun using GPS technology, instead of triangu-lation and/or trilateration, for horizontal positioning. The fact that GPS technology also provided accurate ellipsoidal heights was somewhat overlooked in the 1980s because surveyors, hydrologists, and other users of vertical positions re q u i red or-thometric heights relative to mean sea level, as obtained with tide gauges and spirit leveling, and not geometric heights relative to an abstract mathematical surface (the ellipsoid), as obtained with GPS. The attitude towards using GPS to meas-u re heights gradually evolved, however, as NGS and other institutions developed i m p roved geoidal models for determ i n i n g the spatial separation between mean sea level and the ellipsoid. These impro v e-ments enabled people to convert ellip-soidal heights into orthometric heights with greater and greater accuracy. More-o …