English: This image shows a hypothetical gravity measurement. The red point marked ${\displaystyle g_{m}}$ is the measured gravity at a point on the surface of the earth. The green point is the normal gravity ${\displaystyle g_{n}}$ calculated for that latitude and longitude, which lies on the reference ellipsoid. In this hypothetical example, the reference ellipsoid is located well below the surface point measured. Other influences on the gravity value are the tidal forces of the sun and moon, the gravitational perturbation due to local terrain, and the gravitational perturbation due to a subsurface body of dense rock.