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A tide gauge The tide gauge in Kronstadt, Russia [1] A tide gauge is a device for measuring the change in sea level relative to a vertical datum. [2] [3] It is also known as a mareograph, [4] marigraph, [5] and sea-level recorder. [6] When applied to freshwater continental water bodies, the instrument may also be called a limnimeter. [7] [8]
The PSMSL states that "GLOSS provides oversight and coordination for global and regional sea level networks in support of, and with direction from, the oceanographic and climate research communities." [2] The Global Sea Level Observation System utilizes 290 tide gauge stations and watches over 90 countries and territories to have a global coverage.
For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge. [5] Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out. [6]
The tide gauge data are freely accessible by all, and consist predominantly of monthly-mean and annual-mean sea levels. The primary,"Revised Local Reference" data set has a continuous history of benchmark surveys for each gauge, ensuring that sea level is measured relative to a known land-based datum.
Mean sea level was measured at 26 tide gauges: 21 in the United States and five in Canada.The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment of observations.
Tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level. for the London Underground, a tunnel datum of ODN −100 m is used; [7] thus a depth of −60 m AOD is 40 m ATD (above tunnel datum) for the Channel Tunnel, a tunnel datum of ODN −200 m is used; [8] thus a depth of −60 m is 140 m ATD
Newlyn Tidal Observatory is a grade II listed tide gauge hut on the South Pier in Newlyn, Cornwall in England. Measurements of sea level taken at the observatory between 1915 and 1921 were used to define the reference level, Ordnance Datum Newlyn, for height measurement on the British mainland.
To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level. [3]