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Mean sea level (MSL) is the average height of the sea, with reference to a suitable reference surface. Defining
the reference level [1] is, however, complex and accurately determining
MSL is difficult.
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 - are also averaged out. The
values of MSL are measured with 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 is located.
To extend this definition far from land 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 gravity field. In
reality, due to currents, air pressure variations, temperature and salinity variations etc., this is not the case, 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 surface topography. If varies globally in a range of m.
Traditionally, measurements had to be processed to take into account the effect of the 19-year Metonic cycle and the 223-month Saros cycle on
the tides. Mean sea level is not constant over the surface of the entire earth. Mean sea level at the Pacific end of the Panama
Canal is 20cm higher than at the Atlantic end.
For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise.
Despite the difficulties, aviators using instrument flight rules must have accurate and
reliable measurements of their altitudes above sea level, and the altitude of the airports where they intend to land. That
problem can be compounded when the landing site is an aircraft
carrier in a gravitational anomaly.
On other planets that lack a liquid ocean, a "mean altitude" can be calculated by averaging the heights of all points on the
surface. This altitude is sometimes referred to as a sea level, and is used equivalently as a reference for the height of
planetary features.
UK
In the UK, mean sea level is defined at Newquay in the county of Cornwall.
See also
External links
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