Mean sea level (MSL) is a measure of the average height of the ocean surface (eg the midpoint between high tide and mean low tide); It is used as a criterion for calculating the height of the floor. Mean sea level also plays a role in navigation, as standard sea level pressure is used as a reference to measure the height of flight levels.
For the tide gauge operator, mean sea level means "still water level" - that is, sea level when average wind waves are present - for a certain time period which causes changes in sea level, eg that may be due to the tide as well as with the extraction of the mean. There are scales for measuring mean sea level with respect to the land, where a change in mean sea level can occur as a result of a real change in sea level or a change in the height of the land on which the tide gauges are operated.
In the United Kingdom, war reference point (0 m altitude on UK maps) refers to the mean sea level measured at Newlyn, Cornwall between 1915 and 1921, and prior to 1921 this was the mean sea level at Victoria Harbour, Liverpool .
In France, tides have always been measuring sea level since 1883 and provide the oldest data on sea level, and in part of continental Europe and much of Africa it is used as the official sea level.
Satellite altimeters have been used to provide accurate sea level measurements since the launch of the TOPIX/Poseidon satellite in 1992. In a joint mission between NASA and CNT, the TOPIX/Poseidon satellite of the Jason 1 satellite came in 2001 and a topography mission Ocean surface aboard the Jason 2 satellite in 2008.
Difficulties in use
To extend this definition away from the sea, it means comparing the local height of mean sea level "level" with a reference surface or point, called the geoid. In the case of rest or no external forces, the mean sea level coincides with the surface of the geoid and the surface gravitational potential is equal. In fact, due to currents and variations in air pressure, temperature, salinity, etc., this does not occur, not least as a long-term average. Location dependence, time invariance, and the distinction between mean sea level and geometry are referred to as (invariant) ocean surface relief. It varies globally in the range of ± 2 m.
It is usual to treat mean sea level measurements to take into account the effect of the passage of the sq cycle