en.wikipedia.org
Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.
[4] Instantaneous sea level varies substantially on several scales of time and space. This is because the sea is in constant motion, affected by the tides,
wind, atmospheric pressure, local gravitational differences, temperature,
salinity, and so forth. 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.
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Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time long enough that fluctuations caused by
waves and
tides are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar to
sea level changes (millimetres per year).