What term is the reference level from which tidal height is measured?

• A. Tidal Datum
• B. Tidal Range
• C. Tide
• D. Calcareous ooze

Answer: (A)

Tidal height is measured from a fixed reference surface called a tidal datum. This datum is a long-term average surface—derived from many tides over years—so it provides a stable zero line for comparing water levels. Examples include mean sea level or mean lower low water, used on charts to define elevations and depths. The other terms describe the size of the tides, the tidal phenomenon itself, or a sediment type, none of which provide that standard reference surface.