MLLW is used for soundings and MHW for clearances. How is MLLW defined?

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Multiple Choice

MLLW is used for soundings and MHW for clearances. How is MLLW defined?

Mean Lower Low Water is defined as the average of the lower of the two daily low tides measured over the standard 19-year tidal datum epoch at a given tidal station. In practice, you take the lower low water height each day, collect those values for the 19-year period, and average them. This gives a conservative reference level used for soundings, since depths measured relative to MLLW reflect the lowest normal water level you’d expect. The other options describe different ideas: the highest tide average corresponds to mean high water, the 50-year average sea level relates to mean sea level, and a general mean tide level between high and low isn’t a standard nautical datum.

Mean Lower Low Water is defined as the average of the lower of the two daily low tides measured over the standard 19-year tidal datum epoch at a given tidal station. In practice, you take the lower low water height each day, collect those values for the 19-year period, and average them. This gives a conservative reference level used for soundings, since depths measured relative to MLLW reflect the lowest normal water level you’d expect. The other options describe different ideas: the highest tide average corresponds to mean high water, the 50-year average sea level relates to mean sea level, and a general mean tide level between high and low isn’t a standard nautical datum.

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