Sea level
From Wikipedia, the free encyclopedia.
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 above a 'level' reference surface, or datum, called the geoid.
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.
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UK
In the UK, mean sea level is defined at Newquay in the county of Cornwall.
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