; SI unit metre, m) is a type of height above sea level introduced by the Soviet scientist Mikhail Molodenskii.
The evaluation ranges from the Earth ellipsoid up to the point of interest; the procedure is thus recursive.
Normal heights are slightly dependent upon the reference ellipsoid chosen.
The reference surface where normal heights are zero is called the quasi-geoid (or quasigeoid), a representation of mean sea level similar to the geoid and close to it, but lacking the physical interpretation of an equipotential surface.
In practice, it is determined starting with geodetic levelling and applying correction terms.