The heights determined by GPS measurements refer to which of the following?

• A. The geoid.
• B. A reference ellipsoid.
• C. Sea surface height.
• D. Orthometric height relative to MSL.

Answer: (B)

GPS heights are referenced to a smooth mathematical surface called a reference ellipsoid. The receiver computes ellipsoidal height, which is the distance from this ellipsoid to the receiver along the normal. This height is tied to the chosen datum (like WGS84 or ITRF), not to mean sea level or the geoid. To get height above sea level, you’d apply a geoid model to convert ellipsoidal height to orthometric height using H = h − N, where N is the geoid undulation. Sea surface height is a dynamic quantity tied to tides and ocean dynamics, not what GPS directly measures. So the heights determined by GPS measurements refer to the reference ellipsoid.