For MHW / MLW Coastal Wicki Ref 4 have "The arithmetic mean of the published values of mean high water springs and mean high water neaps." with corresponding definitions for MLW.For MHW and MLW the IHO Ref 2 definition specifies a period of 19 years.For MHWS / MLWS / MHWN / MLWN CoastalWiki Ref 4 add "throughout a year" and "(approximately once a fortnight)".For MHWS / MLWS / MHWN / MLWN the NTLSF Ref 1 add " throughout the year (when the average maximum declination of the moon is 23.5°)".For HAT NOAA Ref 3 have "The elevation of the highest predicted astronomical tide expected to occur at a specific tide station over the National Tidal Datum Epoch." with a corresponding change to LAT. ![]() As defined by United Kingdom National Tidal and Sea Level Facility (NTSLF) Ref 1.MSL: The average observed height of the surface of the sea relative to a stated vertical datum. MTL: The arithmetic mean of mean high water and mean low water. Note 8ĭTL: The arithmetic mean of mean higher high water and mean lower low water. MHLW / MLLW: The average of the higher/lower low water level of each tidal day over a given period. MHHW / MLHW: The average of the higher/lower high water level of each tidal day over a given period. MHW / MLW: The average of all high/low water levels over a given period. MLWS / MLWN: The average of the two successive low waters during those periods of 24 hours when the range of the tide is at its greatest/least. MHWS / MHWN: The average of the two successive high waters during those periods of 24 hours when the range of the tide is at its greatest/least. HAT / LAT: The highest/lowest level that can be expected to occur under average meteorological conditions and under any combination of astronomical conditions. Tidal Levels are often defined as being an average over a time period of one metonic cycle of 19 years or over a lunar nodal cycle of 18.61years. In most places the astronomical tide dominates.īecause of this meteorological variation, HAT is not necessarily the highest possible tide and LAT is not necessarily the lowest possible tide. wind and pressure) and is known as the "Meteorological Tide". The other part of the tide is caused by the weather (e.g. Note that throughout this article, the tide to which we refer is the "Astronomical Tide" - this is that part of the tide caused by the Moon and the Sun. Some tidal levels are more appropriate for diurnal tides, and some are more relevant for semi-diurnal tides. used in setting the height of flood defences. ![]() legally boundaries used to establish territorial, ownership, access and fishing rights. ![]()
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