Orthometric height is defined as height above which surface?

• A. The reference ellipsoid.
• B. The geoid (mean sea level).
• C. The Earth's surface at a specific ocean depth.
• D. A fixed sphere.

Answer: (B)

Orthometric height is the height above the geoid, the equipotential surface that approximates mean sea level. The geoid represents the gravity field’s “level surface” so distances to it are measured along the local vertical, giving a height that corresponds to how high the point sits relative to mean sea level. Because gravity varies across the Earth, the geoid undulates around the reference ellipsoid, which is why orthometric height is not simply the distance above the ellipsoid. In practice, you can relate it to ellipsoidal height h by subtracting the geoid undulation N (H = h − N). Other surfaces like a fixed sphere or an ocean-depth surface don’t reflect the gravity field, so they aren’t used for orthometric height.