In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level.

In Hong Kong, 'mPD' is a surveying term meaning 'metres above Principal Datum' and refers to height of 0.146 m above chart datum and 1.304 m below the average sea level. Since the times of the Russian Empire, in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge. Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool. In the UK, the ordnance datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 19. 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. Global MSL refers to a spatial average over the entire ocean. 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. Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out. 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. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and using it as a datum. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity, and so forth. Instantaneous sea level varies quite a lot on several scales of time and space. Precise determination of a 'mean sea level' is difficult because of the many factors that affect sea level.

Measurement Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year). It causes a significant depression in the Indian Ocean, about 1,200 km (746 mi) southwest of India, where the surface reaches a depth of 106 m (348 ft) below the global mean sea level.

This variation from a perfect sphere is the geoid of the Earth. The term APSL means above present sea level, comparing sea levels in the past with the level today.Įarth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. The term above sea level generally refers to above mean sea level (AMSL).

Because most of human settlement and infrastructure was built in response to a more normalized sea level with limited expected change, populations affected by climate change in connection to sea level rise will need to invest in climate adaptation to mitigate the worst effects or when populations are in extreme risk, a process of managed retreat. When temperatures rise, mountain glaciers and the polar ice caps melt, increasing the amount of water in water bodies.

Current sea level rise is mainly caused by human-induced climate change. Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. 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. 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. Mean sea level ( MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. This marker indicating sea level is situated between Jerusalem and the Dead Sea. For other uses, see Sea level (disambiguation).

36 inch contour gauge