Chart datum reference for tide predictions?

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

Answer: (B)

Chart datum is the zero level used on nautical charts to express depths and tide predictions. The standard reference for tide predictions and sounding data on most charts is Mean Lower Low Water. This choice uses the average of the lower of the two daily low tides over a 19-year cycle, which provides a conservative baseline so depths shown on charts are reliable even at the lowest predictable tides. It helps mariners avoid underestimating shallow areas during peak low-water periods.

Mean Higher High Water would be a high-water reference and isn’t used as the chart’s zero for depths. Mean Sea Level is a general global reference, not the practical chart datum for tide-based depth measurements. Lowest Astronomical Tide is an extreme low-water concept used for safety margins in some contexts, but it’s not the standard chart datum for routine tide predictions.