Navigation, Seamanship, and Shiphandling (NSS) – Officer of the Deck (OOD) Phase 2 Practice Exam

A consistent, conservative depth datum is used on nautical charts to ensure safe navigation across tides. Mean Lower Low Water represents the average of the lower of the two daily low tides, so charted depths are tied to the lowest regular water level mariners can expect. Using this datum provides a safety margin: the depth shown is effectively the minimum depth you’re likely to encounter during ordinary low tide, helping you plan under-keel clearance to avoid grounding. Other datums can be more extreme or less practical for routine use, so the standard choice is to base depths on this lowest regular tide to keep navigation conservatively safe.