Mean Sea Level at a station is the plain average of every hourly water level recorded there through the tidal datum epoch. It is a local tidal datum, not a single global surface, and it differs from place to place by tens of centimetres because of winds, currents, water density and land motion. Monthly and annual means of the same quantity are the raw material for sea level trend analysis.
Local MSL is 0.3 feet above the vertical land datum here, so don't treat them as the same zero.
Height above sea level means little at the coast unless the datum is named; local mean sea level and the land vertical datum can differ by tens of centimetres.
Also heard as
- mean sea level
- LMSL
- local mean sea level
Do not mix it up with
- MTL: Mean tide level is the midpoint of mean high and mean low water; mean sea level averages every hourly height, and the two differ where the tide curve is lopsided.
Related terms
MTL
Mean Tide Level: the height midway between the mean high water and mean low water datums.
NAVD
North American Vertical Datum: the geodetic height reference used for land elevations across the United States.
Relative sea level trend
The long-term rate of change in mean sea level at a gauge, including both ocean rise and land motion.
Tidal datum
A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.