The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established for vertical control surveying in the United States of America based upon the General Adjustment of the North American Datum of 1988.
The NAVD 88 was established in 1991 by the minimum-constraint adjustment of geodetic leveling observations in Canada, the United States, and Mexico. It held fixed the height of the primary tidal bench mark, referenced to the International Great Lakes Datum of 1985 local mean sea level height value, at Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks.
The NAVD 88 replaced the National Geodetic Vertical Datum of 1929 (NGVD 29), previously known as the Sea Level Datum of 1929. It is important to note that, while the computed elevation of some fixed point might differ depending on which data us used, the elevation of that point relative to some other near by point did not change with that datum change.