National Geodetic Vertical Datum of 1929 (NGVD29) 1
National Geodetic Vertical Datum of 1929 (NGVD29) 1 : A fixed reference adopted as a standard geodetic datum for elevations determined by leveling. The datum was derived for surveys from a general adjustment of the first-order leveling nets of both the United States and Canada. In the adjustment, mean sea level was held fixed as observed at 21 tide stations in the United States and 5 in Canada. The geodetic datum now in use in the United States is the National Geodetic Vertical Datum. The year indicates the time of the general adjustment. The geodetic datum is fixed and does not take into account the changing stands of sea level. Because there are many variables affecting sea level, and because the geodetic datum represents a best fit over a broad area, the relationship between the geodetic datum and local mean sea level is not consistent from one location to another in either time or space. For this reason, the National Geodetic Vertical Datum should not be confused with mean sea level. NGVD (1929) has been superseded for use by NAVD88. (Seewww.ngs.noaa.gov/faq.shtml)
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