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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, referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) height value, at Rimouski, Quebec, Canada.
Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD 29) May 10, 1973, by the National Geodetic Survey , a part of the National Oceanic and Atmospheric Administration .
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [1] Common chart datums are lowest astronomical tide (LAT) [1] and mean lower low water (MLLW).
The mean sea level at a particular location may be calculated over an extended time period and used as a datum. For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge. [5]
Countries tend to choose the mean sea level at one specific point to be used as the standard "sea level" for all mapping and surveying in that country. (For example, in Great Britain, the national vertical datum, Ordnance Datum Newlyn, is based on what was mean sea level at Newlyn in Cornwall between 1915 and 1921). [3]
Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. [14] During the glacial-interglacial cycles over the past few million years, the mean sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay ...
In North America elevations are given using either Sea Level Datum of 1929, also called the National Geodetic Vertical Datum of 1929 (NGVD 29) or the North American Vertical Datum of 1988 (NAVD 88).
The orthometric height (symbol H) is the vertical distance along the plumb line from a point of interest to a reference surface known as the geoid, the vertical datum that approximates mean sea level. [1] [2] Orthometric height is one of the scientific formalizations of a layman's "height above sea level", along with other types of heights in ...