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The North American Vertical Datum of 1988 (NAVD 88) is the vertical datum for orthometric heights established for vertical control surveying in the United States of America based upon the General Adjustment of the North American Datum of 1988.
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 tide gauge benchmark (surveying), referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) 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., that MSL is not the same equipotential surface at all tidal bench marks.
The definition of NAVD 88 uses the Helmert orthometric height, which calculates the location of the geoid (which approximates MSL) from modeled local gravity. The NAVD 88 model is based on then-available measurements, and remains fixed despite later improved geoid models.
NAVD 88 replaced the National Geodetic Vertical Datum of 1929 (NGVD 29), previously known as the Sea Level Datum of 1929. The elevation difference between points in a local area will show negligible change from one datum to the other, even though the elevation of both does change. NGVD 29 used a simple model of gravity based on latitude to calculate the geoid and did not take into account other variations. Thus, the elevation difference for points across the country does change between datums.