NGS derived the original realization of the North American Datum of 1983 NAD 83 in 1986
by performing a rigorous adjustment of most of the classical geodetic observations in its archives
together with Doppler observations and a few very long baseline interferometry VLBI baselines Schwarz 1989.
This original realization is called NAD 83 1986. With improvements in our knowledge of terrestrial reference frames,
NGS has introduced several newer realizations of NAD 83, refining at each step the adopted coordinates.
In 1998 NGS introduced the current realization,