Sketch of the gravity and normal gravity vectors on the Earth's surface, the geoid, and the reference ellipsoid

Sketch of three surfaces encountered in gravimetry: the Earth's surface, the geoid (an equipotential of the Earth's gravity field), and the reference ellipsoid (a geometric approximation of the Earth). The Earth's gravity vector (g) and the normal gravity vector (γ, the gravity of the reference ellipsoid) are shown intersecting these surfaces. The difference between the gravity and normal gravity vectors at point P (on the surface) is the gravity disturbance vector δgP. Also shown: the geometric height (h), the orthometric height (H), and the geoid height (N).

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