Question

Which one of the following statements is CORRECT?

• A. The mean sea level is not the same as the geoid, but an approximation to the geoid
• B. At least two tide gauges are required for estimating the mean sea level
• C. The reference ellipsoid is always below the geoid
• D. A geoid is realized by taking the mean of the tide gauge measurements at a point over a long period of time (≥ 1 year)

Hint:

Recall that mean sea level differs from the geoid because of sea surface topography, and think about how national height datums and geoid undulation actually behave.

Answer 1

The Correct Option is A

Step 1: Recall the definitions of geoid and mean sea level (MSL).
The geoid is the equipotential surface of the Earth's gravity field that best fits, in a least-squares sense, the undisturbed mean sea level over the entire oceans. It is a physically defined surface based on the Earth's actual gravity field. Mean sea level, on the other hand, is the average sea surface height observed at a tide gauge over a sufficiently long period.
Step 2: Examine option (A).
Because of oceanographic effects such as currents, tides, wind stress, salinity and temperature variations, and atmospheric pressure loading, the average sea surface (sea surface topography) departs from the geoid by up to about 1 to 2 metres in places. Hence mean sea level is close to, but not exactly, the geoid, it is only an approximation to it. This statement is correct.
Step 3: Examine option (B).
Mean sea level at a location can be estimated from the record of a single tide gauge averaged over a long enough period (conventionally about 18.6 years, to average out the lunar nodal cycle). A single station suffices for a local MSL estimate, so requiring at least two tide gauges is not a necessary condition. This statement is incorrect.
Step 4: Examine option (C).
The reference ellipsoid is a smooth mathematical surface fitted to approximate the geoid globally. The geoid undulation (separation between geoid and ellipsoid) is sometimes positive and sometimes negative depending on location, so the ellipsoid is not always below the geoid. This statement is incorrect.
Step 5: Examine option (D).
Averaging tide gauge measurements over a long period gives the mean sea level at that location, not the geoid itself. The geoid is a gravity equipotential surface computed from gravimetric and satellite data over the whole Earth, so a single tide-gauge time average only realizes local MSL, which is itself merely an approximation to the geoid. This statement is incorrect.
Step 6: Select the correct statement.
Only option (A) is correct.\[ \boxed{\text{Mean sea level is an approximation to the geoid, not identical to it}} \]