Which statement best describes tidal datums?

• A. Determined by averaging the level of water at a tide gage over time.
• B. Are fixed heights above mean sea level.
• C. Include Mean Sea Level only.
• D. Are derived from gravity measurements.

Answer: (A)

Tidal datums are reference surfaces created from long-term observations of the water surface at a coastal tide gauge. By averaging the water levels over many years, typically capturing the full range of tidal and seasonal variations, a stable datum is established for that specific location. This approach reflects how tides behave locally and provides a consistent height reference used in surveying and mapping. There are multiple tidal datums used for different purposes (for example mean low water, mean high water, mean sea level, etc.), not just a single fixed value. They are not fixed heights above mean sea level, and they aren’t derived from gravity measurements—the data come from observed water levels, not the gravity field.