The zero point a height is measured from. A seawall height is meaningless without it: five feet above mean low water and five feet NAVD88 are different elevations. Some datums are geodetic, like NAVD88, tied to a national network of survey marks. Others are tidal, defined by the tide itself. NOAA defines mean low water as the average of all the low water heights observed over the National Tidal Datum Epoch, a specific 19-year period. South Florida codes use several. Most cities we have verified state their rule in NAVD88, Coral Gables states NGVD29, Key Colony Beach states mean sea level, and Surfside ties its minimum to the FEMA base flood elevation. We publish each as written and do not convert between them.
Source: NOAA CO-OPS, Datum definitions · Checked 2026-10
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