Which surface is defined as mean sea level extended through the continents and used as a reference for orthometric heights?

• A. Ellipsoid
• B. Geoid
• C. Spheroid
• D. Datum

Answer: (B)

The surface in question is the geoid. It represents the shape that mean sea level would take if extended across the continents, governed by Earth’s gravity field as an equipotential surface. Orthometric heights are the vertical distances above this surface, so they measure “height above the geoid.” This makes the geoid the natural reference for true elevations, since it follows gravity and the actual distribution of mass beneath the surface, unlike a smooth geometric model.

By contrast, an ellipsoid (or spheroid) is a smooth mathematical approximation of Earth’s shape used for positioning and map projections, not a gravity-following reference surface. A datum defines a coordinate system and reference frame (and may specify a vertical reference), but the surface used for orthometric heights is specifically the geoid.