NAVD 88 or NGVD 29, what is the difference between them?
“The National Geodetic Vertical Datum of 1929″ (NGVD29) which is the name, after May 10, 1973, of (the) Sea Level Datum of 1929.” “Sea Level Datum of 1929: A vertical control datum established for vertical control in the United States by the general adjustment of 1929.” “Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the U.S.A. and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. A total of 106,724 km of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level.” “The datum (was) not mean sea level, the geoid, or any other equipotential surface. Therefore it was renamed, in 1973, the National Geodetic Vertical Datum on 1929.” The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican-U.S. leveling observations.
