Leveling-based datums versus gravimetric datums: which statement is correct?

• A. Leveling-based datums are tied to physical level surfaces via spirit leveling; gravimetric datums are derived from gravity measurements and geoid models, with different zero-height definitions.
• B. Leveling-based datums use gravity measurements; gravimetric datums rely on spirit leveling.
• C. Leveling-based is for horizontal positions; gravimetric is for vertical.
• D. Leveling-based datums and gravimetric datums are identical but named differently.

Answer: (A)

Leveling-based datums establish elevations by physically leveling to a reference surface. Spirit leveling measures height differences along a network and ties the zero-height to a level surface that approximates mean sea level. Gravimetric datums define height from the Earth's gravity field using a geoid model; they assign height relative to a geopotential surface, so the zero height is defined by a chosen potential value and can differ from mean sea level. Because of that, the two systems produce different heights for the same point and are not interchangeable. The other statements misstate how leveling and gravimetric datums are formed, mix up their roles, or claim they are identical.