Reference surfaces in geodesy and levelling: The slightly irregular, curved surface that represents mean sea-level extended under the continents (zero potential surface) is called the ______.

Introduction / Context:
Accurate height systems require a physically meaningful reference. In geodesy and levelling, the geoid approximates mean sea level (MSL) globally, extended under land. It is an equipotential surface of Earth’s gravity field and provides the basis for orthometric heights and spirit levelling reductions.

Given Data / Assumptions:

• The question seeks the name of the MSL-based zero-elevation surface.
• Minor local undulations are expected due to gravity variations.
• Surveying terminology distinguishes geometric, physical, and mathematical surfaces.

Concept / Approach:

The geoid is the equipotential surface that best fits global mean sea level. It is irregular because Earth’s mass distribution is not uniform. A level surface is any equipotential; the geoid is a particular one chosen as reference. A horizontal surface is tangent to a level surface at a point (a local plane). The reference ellipsoid is a smooth mathematical figure approximating Earth for map projections and geodetic computations, not the physical zero-height surface.

Step-by-Step Solution:

Verification / Alternative check:

Modern geoid models (e.g., EGM family) represent this surface globally. Survey control networks use geoid undulations to convert between ellipsoidal and orthometric heights.

Why Other Options Are Wrong:

Mean sea level describes an observation, not the full global equipotential; a level surface is generic; a horizontal surface is local; the ellipsoid is a smooth reference used mainly for positions, not physical zero height.

Common Pitfalls:

Equating ellipsoidal heights (GNSS) directly with orthometric heights without applying geoid undulation; treating MSL at one tide gauge as the universal zero.

Final Answer:

Geoid surface