Sea level

Mean sea level (MSL) (often shortened to sea level) is an average level of the surface of one or more of Earth's bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. However 20th century and current millennium sea level rise is caused by global warming, and careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level generally refers to above mean sea level (AMSL). The term APSL means Above Present Sea Level, comparing sea levels in the past with the level today. Precise determination of a 'mean sea level' is difficult to achieve because of the many factors that affect sea level. Instantaneous sea level varies quite a lot on several scales of time and space. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point. Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out.Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the tides, also have zero mean. Global MSL refers to a spatial average over the entire ocean. One often measures the values of MSL in respect to the land; hence a change in relative 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, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the vertical datum was MSL at the Victoria Dock, Liverpool.Since the times of the Russian Empire, in Russia and other former its parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. In Hong Kong, 'mPD' is a surveying term meaning 'metres above Principal Datum' and refers to height of 1.230m below the average sea level.In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references (European Vertical Reference System) are made to the Amsterdam Peil elevation, which dates back to the 1690s. 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. Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to the average sea level datum. It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with flight level), and in the atmospheric sciences, and land surveying. An alternative is to base height measurements on an ellipsoid of the entire Earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and mean tidal height. The alternative is to use a geoid-based vertical datum such as NAVD88 and the global EGM96 (part of WGS84).