Mean lower low water (MLLW) is defined as?

• A. Reference level for depths and heights
• B. Average of lowest daily tides, chart datum
• C. Water depth in feet, fathoms, or meters
• D. Difference between true north and magnetic north

Answer: (B)

Mean lower low water is the standard water level used for charting, defined as the average of the lower low water heights observed over a long period. In practice, you take the lower of the two daily low tides (the lowest dip each day) and average those values over many tides, so the resulting datum is used as the reference for depths on charts. This is why the option describing the average of the lowest tides and noting chart datum fits best. It’s not just a general reference level for depths, not a simple depth measurement, and not related to magnetic variation.