Which tidal datum is used for depths?

Master the Navigation, Seamanship, and Shiphandling (NSS) – Officer of the Deck (OOD) Phase 2 Exam with engaging flashcards and multiple choice questions, complete with hints and explanations. Prepare comprehensively for your test!

Multiple Choice

Which tidal datum is used for depths?

Explanation:
Depths on navigational charts are referenced to a fixed chart datum, chosen to provide a conservative baseline for safe navigation. The datum used for depths is Mean Lower Low Water—the average of the lower of the two low tides each tidal day, averaged over the 19-year cycle. Using this lowest-tide reference ensures that the depth shown on the chart accounts for the most adverse predictable low-water conditions, so vessels won’t run aground as the tide falls. The other datums describe different tide levels (high-water references or overall sea level) and aren’t used as the standard basis for depth measurements.

Depths on navigational charts are referenced to a fixed chart datum, chosen to provide a conservative baseline for safe navigation. The datum used for depths is Mean Lower Low Water—the average of the lower of the two low tides each tidal day, averaged over the 19-year cycle. Using this lowest-tide reference ensures that the depth shown on the chart accounts for the most adverse predictable low-water conditions, so vessels won’t run aground as the tide falls. The other datums describe different tide levels (high-water references or overall sea level) and aren’t used as the standard basis for depth measurements.

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