Mean sea level is defined as which of the following?

• A. The current sea level at a location.
• B. The average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period.
• C. The height of the sea relative to land during summer.
• D. The surface reference for land altitude.

Answer: (A)

Mean sea level is the long-term average height of the sea surface at a location, determined by averaging measurements of the sea surface height over all stages of the tide across a 19-year period. This 19-year span accounts for the regular lunar-solar tidal cycle, giving a stable reference value unaffected by daily tides, weather, or short-term fluctuations. That makes it the appropriate basis for measuring land elevation and for altitude references used in aviation and mapping.

The current sea level at a location can vary above or below this average, so it isn’t the definition of mean sea level. The idea of height relative to land during summer doesn’t capture the tidal averaging involved. And while mean sea level serves as a reference for land altitude, the definition centers on the averaged sea surface height itself, not the reference role.