🔍 Key Concepts

• Tidal datums such as mean high water, mean low water, and how they are defined • The difference between higher high water and lower high water in a semidiurnal (two‑high‑tide) day • Whether a definition uses all occurrences of a tide level or only one of the daily highs or lows

💭 Think About

• In an area with two high tides each day, think about whether mean high water would ignore one of them or include both in the average. • Look at which choice sounds like a statistical average of a data set, versus which ones sound like they are picking only one specific tide each day. • Ask yourself: if I were a hydrographer making charts, would I want my reference level to be based on just some of the high waters, or on the complete set of high waters over the observation period?

✅ Before You Answer

• Make sure the option you pick could logically be used as a chart datum along coasts with mixed or semidiurnal tides. • Check whether the wording limits you to only the higher or only the lower daily highs—that may leave out some of the tide data. • Confirm that your choice reflects an average over a long period (usually many years of records), not a single daily event.