The concept of “sea level” is a curious beast because the oceans aren’t level.
This irregular “sea level” is also continuously moving.
However, after about nineteen years it is possible to calculate the Local Mean Sea Level for a given location provided everything else remains constant.
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 (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out.
Nineteen years is preferred because the Earth, moon and sun’s relative positions repeat almost exactly in the Metonic cycle of 19 years, which is long enough to include the 18.613 year lunar nodal tidal constituent.
Unfortunately, not everything remains constant.
The Earth’s surface rises and falls vertically.
The Earth’s crust stretches and compresses horizontally.
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