It is used, among other things as a datum from which to measure the navigational clearance, or air draft, under bridges. Similarly, the mean higher high water (MHHW) is the average height of the highest tide recorded at a tide station each day during the recording period. This is the datum used for coastal charts published by the Canadian Hydrographic Service, with the average taken from 19 years of tidal predictions. This is an average of lowest low waters taken over a fixed period of tidal predictions, as opposed to actual observations. The 19-year recording period is the nearest full year count to the 18.6-year cycle of the lunar node regression, which has an effect on tides.
MLLW is only a mean, so some tidal levels may be negative relative to MLLW see also #Mean low water springs. Mean lower low water (MLLW) is the average height of the lowest tide recorded at a tide station each day during a 19-year recording period, known as the National Tidal Datum Epoch as used by the United States' National Oceanic and Atmospheric Administration. Lunar month (the springs) and the lowest each lunar month (the Neaps) averaged over the year.' Mean water Mean lower low water In Australia, the definition of the MHW is '.the line of the medium high tide between the highest tide of each In the United States this period spans 19 years and is referred to as the National Tidal Datum Epoch. It is not the same as the normal tidal limit. Mean high water (MHW) is the average of all the daily tidal high water levels observed over a period of several years. The highest astronomical tide (HAT) can be defined similarly. One advantage of using LAT for chart datums is that all predicted tidal heights must then be positive (or zero) avoiding possible ambiguity and the need to explicitly state sign.Ĭalculation of the LAT only allows for gravitational effects so lower tides may occur in practice due to meteorological effects, such as high pressure systems. Many national charting agencies, including the United Kingdom Hydrographic Office and the Australian Hydrographic Service, use the LAT to define chart datums. Lowest astronomical tide (LAT) is defined as the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. The following tidal phases are commonly used in the definition of chart datums.
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