Which tidal datum is the average of all lower water heights over a 19-year cycle?

• A. Mean Sea Level
• B. Mean High Water
• C. Mean Lower Low Water
• D. Lowest Astronomy Tide

Answer: (C)

Tidal datums are fixed reference levels used for charting and navigation, derived from measuring tides over long periods. The datum described here is defined as the average of the lower water heights across a 19-year cycle. That means you take the two low tides each tidal day, pick the lower one, and average all those lower-low water measurements over the 19-year period (the lunar nodal cycle). The 19-year span smooths out the monthly and orbital variations so the datum is stable and repeatable for depth measurements.

This fits best because it specifically targets the lower low water heights and the long, 19-year averaging cycle. Other datums serve different purposes: Mean Sea Level is the overall average sea surface, not limited to the lower lows; Mean High Water is the average of the higher tides; and Lowest Astronomy Tide is a theoretical minimum tide, not an average taken over the lunar cycle.