Datums

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Datums

What is a datum?

It is simply a reference level, a zero surface to which storm surge heights are referred.

There are many several different datums:

Tidal datums: reference water levels defined by a phase of the tide.

Mean Sea Level (MSL)
Mean Lower Low Water (MLLW)
Mean Higher High Water (MHHW)

Geodetic datums: based on a simplified surface that represents the Earth’s shape and size.

North American Vertical Datum of 1988 (NAVD88)
National Geodetic Vertical Datum of 1929 (NGVD29)

Inundation is the total water level that occurs on normally dry ground as a result of storm tide, and is expressed in terms of height of water, in feet, above ground level (AGL). Inundation provides the most clearly and commonly understood method for communicating storm surge driven coastal flooding. The tidal datum MHHW (Mean Higher High Water) is the best possible approximation of the threshold at which inundation can begin to occur. Along the coast, areas higher than MHHW are typically dry most of the time.

Knowing your elevation and using a consistent datum is very important. When comparing water level observations (from tide stations) with other observations, observations with models or models to other models, you must always use consistent vertical datums in order to avoid misinterpreting surge elevations.

Tidal Datums