Why did NGS change from NAD 27 to NAD 83?
NAD 83 was computed by the geodetic agencies of Canada (Federal and Provincial) and the National Geodetic Survey for several reasons. The horizontal control networks had expanded piecemeal since 1933 to cover much more of the countries and it was very difficult to add new surveys to the network without altering large areas of the previous network. Field observations had added thousands of accurate Electronic Distance Measuring Instrument (EDMI) base lines, hundreds of additional points with astronomic coordinates and azimuths, and hundreds of Doppler satellite determined positions. It was also recognized that the Clarke Ellipsoid of 1866 no longer served the needs of a modern geodetic network. For an in-depth explanation see NOAA Professional Paper NOS 2 “The North American Datum of 1983”, Charles R. Schwarz, Editor, National Geodetic Survey, Rockville, MD 20852, December 1989. • What is the relationship between the geodetic vertical datums (NGVD 29 and/or NAVD 88) and the various wate
