What is Mean Lower Low Water?
Mean lower low water (abbreviated MLLW) is a tidal datum, or reference level against which the tide height is measured. It is the average of the lower of the two low water heights of each tidal (or lunar) day over a reference 19 year period. A 19 year period was chosen since it represents a full lunar cycle. Every 19 years, the moon, earth and sun are in essentially the same relative positions, and so, all the possible sun-earth-moon configurations should occur in a 19 year period.
