Sea level
Mean sea level (MSL) is the average (mean)
height of the sea, with reference to a suitable
reference surface. Defining the reference
level’, however, involves complex measure-
ment, and accurately determining MSL can
prove difficult.[1]
Measurement
Sea level measurements from 23 long tide
gauge records in geologically stable environ-
ments show a rise of around 20 centimeters
(8 inches) during the 20th century (2 milli-
meters/year).
To an operator of a tide gauge, MSL means
the "still water level"—the level of the sea
with motions such as wind waves averaged
out—averaged over a period of time such that
changes in sea level, e.g., due to the tides,
also get averaged out. One measures the val-
ues of MSL in respect to the land. Hence a
change in MSL can result from a real change
in sea level, or from a change in the height of
the land on which the tide gauge operates.
In the UK, mean sea level has been meas-
ured at Newlyn in Cornwall and Liverpool on
Merseyside for decades, by tide gauges to
provide Ordnance Datum for the zero metres
height on UK maps.
Difficulties in utilization
To extend this definition far from the sea
means comparing the local height of the
mean sea surface with a "level" reference
surface, or datum, called the geoid. In a state
of rest or absence of external forces, the
mean sea level would coincide with this geoid
surface, being an equipotential surface of the
Earth’s gravitational field. In reality, due to
currents, air pressure variations, temperat-
ure and salinity variations, etc., this does not
occur, not even as a long term average. The
location-dependent, but persistent in time,
separation between mean sea level and the
geoid is referred to as (stationary) sea sur-
face topography. It varies globally in a range
of ± 2 m.
Traditionally, one had to process sea-level
measurements to take into account the effect
of the 228-month Metonic cycle and the 223--
month eclipse cycle on the tides. Mean sea
level does not remain constant over the sur-
face of the entire earth. For insta