How is Mean High Water measured?

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

How is Mean High Water measured?

Mean High Water is the average of the high-water heights observed over a long standard period, so that a stable reference level emerges from the natural variability of tides. In practice, this uses the 19-year tidal epoch: high-water readings are taken at every high tide throughout the 19 years, and all those elevations are averaged to produce MHW. This long record smooths out the monthly, seasonal, and long-term fluctuations caused by the Moon’s orbit and the Sun, giving a consistent datum for charts and navigation. What you’d expect is that you’re averaging many high-water events over years, not averaging water heights within a single day or a short span like 19 days. The other options describe methods that don’t reflect how MHW is defined—using a daily maximum, or averaging over a short period, or focusing on the lower low water rather than the high water.

Mean High Water is the average of the high-water heights observed over a long standard period, so that a stable reference level emerges from the natural variability of tides. In practice, this uses the 19-year tidal epoch: high-water readings are taken at every high tide throughout the 19 years, and all those elevations are averaged to produce MHW. This long record smooths out the monthly, seasonal, and long-term fluctuations caused by the Moon’s orbit and the Sun, giving a consistent datum for charts and navigation.

What you’d expect is that you’re averaging many high-water events over years, not averaging water heights within a single day or a short span like 19 days. The other options describe methods that don’t reflect how MHW is defined—using a daily maximum, or averaging over a short period, or focusing on the lower low water rather than the high water.

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