What does Mean Lower Low Water stand for (MLLW)?

• A. Mean Higher High Water (MHHW)
• B. Mean Lower Low Water (MLLW)
• C. Mean Sea Level (MSL)
• D. Lowest Astronomical Tide (LAT)

Answer: (B)

Mean Lower Low Water is the vertical datum used on nautical charts to set the zero reference for depths. It is defined as the average of the lower of the two daily low tides over the 19-year cycle used to establish tidal data. This means chart depths are based on what you typically see at the lowest tides, providing a stable reference for navigation and shoreline planning.

Other datums differ: Mean High Water is the average of the higher daily high tides, Mean Sea Level is a global average not tied to a specific site’s tides, and Lowest Astronomical Tide is the lowest tide that can be predicted under average astronomical conditions. These distinctions help explain why the term specifically stands for Mean Lower Low Water.