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 measurement, and accurately determining MSL can prove difficult.
[1]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 values 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 measured 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.
Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008.
