Mean sea level (MSL)

Mean sea level, MSL, is the average height of the sea surface at a place over a long period, long enough to average out waves, tides, and weather, traditionally the 19-year cycle that spans one full nodal period of the tide. It is the datum to which land heights and atmospheric pressure are reduced, and the reference from which chart datum is defined: chart datum is set a fixed amount below MSL, at or near the lowest astronomical tide. MSL is realized at a tide gauge as the mean of recorded water levels and tied to the national leveling network. It varies from place to place and rises slowly with time, so a stated epoch matters.