FLORIDA GEOLOGICAL SURVEY
was assigned to the points on mean sea Epoch (i.e., the Metonic cycle; shorter series level determined at the following 26 tide are appropriately named, e.g., Monthly Mean stations. Sea Level, etc.). It was not until 1973 that the confusion over the Sea Level Datum or
Father Point, Quebec St. Augustine, Fla. "Mean Sea Level" as it popularly came to be
Halifax, Nova Scotia Cedar Keys, Fla. known and Mean Water Level was resolved
Yarmouth, Nova Scotia Pensacola, Fla. known and Mean Water Level was resolved Portland, Me. Biloxi, Miss. by assigning the more appropriate name of Boston, Mass. Galveston, Tex. "National Geodetic Vertical Datum of 1929"
Perth Amboy, N.J.' San Diego, Calif. (NGVD) to replace "Sea Level Datum of Atlantic City, N.J. San Pedro, Calif. 1929". NGVD of 1929 is additionally
Baltimore, Md. San Francisco, Calif.
Baltimore, Md. San Francisco, Calif. defined (Harris, 1981) as a fixed reference
Annapolis, Md. Fort Stevens, Ore.
Old Point Comfort, Va. Seattle, Wash. adopted as a standard geodetic datum for Norfolk, Va. Anacortes, Wash. elevations determined by leveling. It does Brunswick, Ga. Vancouver, not take into account the changing stands of British Columbia sea level. Because there are many variables Femrnandina, Fla. Prince Rupert, affecting sea level, and because the geodetic British Columbia
Bih Columbia datum represents a best fit over a broad
'There was no tide station at Perth area, the relationship between the geodetic
Amboy, but the elevation of a bench mark at datum and local mean sea level is not Perth Amboy was established by leveling consistent from one location to another in from the tide station at Sandy Hook. either time or space. For this reason NGVD
The 929 adjustment provided should not be confused with mean sea level,
The 1929 adustment provided the
basis for the definition of elevations even though it has always been defined by a
throughout the nationalnetwork asit existed mean sea level (Schomaker, 1981).
in 1929, and the resulting datum is st used
today. The various North American tidal datum planes are defined (e.g., Mariner, 1951;
The elevation adjustment of 1929 was Swanson, 1974; U. S. Department of referred to as the "Sea Level Datum of 1929", Commerce, 1976; Anonymous, 1978; although it commonly became known as the Harris, 1981; Hicks, 1984) as follows: "Mean Sea Level". In coastal work, however,
there are two standard Design Water Levels National Tidal Datum Epoch the specific 19(DWLs) that are applied. These and their year period adopted by the National Ocean definitions (Galvin, 1969) are: Service as the official time segment over which tide observations are taken and reduced
Mean Water Level (MWL) the time-averaged to obtain mean values for tidal datums. It is water level in the presence of waves, and necessary for standardization because of
necessary for standardization because of
periodic and apparent secular trends in sea
Still Water Level (SWL) the time-averaged level. It is reviewed annually for possible water level that would exist if the waves are revision and must be actively considered for stopped but the astronomical tide and storm revision every 25 years. surge are maintained.
Mean Higher High Water (MHHW) the
These water levels (i.e., MWL and SWL) average of the higher high water heights of
appl fo an legthof tme verwhih a average of the higher high water heights of apply for any length of time over which a each tidal day observed over the National field study or experiment is conducted, while Tidal Datum Epoch. Mean Sea Level and other tidal datums are
determined as an average of measurements Mean High Water (MHW) the average of all made over the 19-year National Tidal Datum the high water heights observed over the
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