Mean sea level is the average height of the sea surface over what period?

• A. The current sea level at a location.
• B. The height of the sea surface relative to land at a single tide.
• C. The average height of the sea surface over a 19-year period.
• D. The lowest level of the tide.

Answer: (C)

Mean sea level is the long-term reference height of the sea surface, used for elevations and charts. To get a stable value, the sea surface is averaged over a full tidal cycle that repeats over time, specifically a 19-year period. This tidal epoch captures the complete range of tides driven by the Sun and Moon, including the variations from spring to neap tides and nodal effects, while smoothing out short-term fluctuations from weather and currents. So the mean sea level is defined as the average height of the sea surface over that 19-year period.