The geoid is the equipotential surface of the Earth's gravity field that best fits mean sea level in a least-squares sense. Which mean sea level is used?

The geoid is the equipotential surface of the Earth's gravity field that best fits mean sea level in a least-squares sense. Which mean sea level is used?

• A. Regional
• B. Local
• C. Global
• D. National

Answer: (C)

A single, globally representative mean sea level is used to define the geoid because the geoid is a global surface tied to Earth’s gravity field. To obtain a consistent reference that works over the entire planet, the mean sea surface must be averaged across the world’s oceans rather than just in a local or regional area. Local or regional mean sea level fluctuates with tides, ocean dynamics, winds, and pressure variations, so using them would bias the geoid toward regional conditions and produce multiple incompatible surfaces. National or regional baselines would be even more biased and nonuniform as a global reference. The global mean sea level provides a single, worldwide dataset that, when fit to the gravity field in a least-squares sense, yields a coherent geoid surface applicable across both oceans and continents.

A single, globally representative mean sea level is used to define the geoid because the geoid is a global surface tied to Earth’s gravity field. To obtain a consistent reference that works over the entire planet, the mean sea surface must be averaged across the world’s oceans rather than just in a local or regional area. Local or regional mean sea level fluctuates with tides, ocean dynamics, winds, and pressure variations, so using them would bias the geoid toward regional conditions and produce multiple incompatible surfaces. National or regional baselines would be even more biased and nonuniform as a global reference. The global mean sea level provides a single, worldwide dataset that, when fit to the gravity field in a least-squares sense, yields a coherent geoid surface applicable across both oceans and continents.