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 interferometry (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.