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.
The mean sea level is not the same as the geoid, but an approximation to the geoid
At least two tide gauges are required for estimating the mean sea level
The reference ellipsoid is always below the geoid
A geoid is realized by taking the mean of the tide gauge measurements at a point over a long period of time (≥ 1 year)
Show Solution
The Correct Option isA
Solution and Explanation
Step 1: Use elimination by checking each claim against a known geodetic fact. Instead of starting from definitions, test each statement and cross it out if it fails a known result. Step 2: Cross out option (C). Geoid undulation $N$ (height of geoid above the ellipsoid) ranges roughly from about $-107$ m near Sri Lanka to about $+85$ m near New Guinea. Since $N$ takes both signs across the globe, the ellipsoid cannot be always below the geoid. Option (C) is eliminated. Step 3: Cross out option (D). A long-term tide gauge average defines local mean sea level, which is a height datum, not the geoid. The geoid is a globally consistent equipotential surface derived from gravimetric, satellite altimetry and gravity-mission data, so calling a single tide-gauge time average a realization of the geoid is inaccurate. Option (D) is eliminated. Step 4: Cross out option (B). National height datums have historically been fixed using a single benchmark tide gauge. A minimum of two gauges is not required for estimating a mean sea level value at a place. Option (B) is eliminated. Step 5: Confirm the remaining option (A). The classical Gauss-Listing definition ties the geoid to mean sea level only as a best least-squares fit; the actual sea surface differs from the geoid because of sea surface topography, so MSL approximates the geoid but is not identical to it.\[ \boxed{\text{Option (A)}} \]
