Normal height explained
Normal heights (symbol
or
;
SI unit metre, m) is a type of height
above sea level introduced by the Soviet scientist
Mikhail Molodenskii.The normal height of a point is defined as the quotient of a point's geopotential number
C (i.e. its geopotential difference with that of sea level), by the vertically averaged
normal gravity:
The average is evaluated along the normal potential's
plumb line (a curve, approximated by the
ellipsoidal normal, a straight line). 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.
Normal gravity values are easier to compute compared to actual gravity, as one does not have to know the Earth's crust density. This is an advantage of normal heights compared to orthometric heights.
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. The geoid undulation
with respect to the reference ellipsoid,
, finds an analogue in the so-called
height anomaly,
:
The
geoid–quasigeoid separation (GQS),
, is zero over the oceans and maximum in the
Himalayas, where it attains approximately 5 meters.
[1] [2] Russia and many other Eastern European countries have adopted a height system based on normal heights. In practice, it is determined starting with geodetic levelling and applying correction terms.
Alternatives to normal heights include orthometric heights (geoid-based) and dynamic heights.
See also
Notes and References
- Sjöberg . Lars E. . On the geoid and orthometric height vs. quasigeoid and normal height . Journal of Geodetic Science . 1 December 2018 . 8 . 1 . 115–120 . 10.1515/jogs-2018-0011 . 2018JGeoS...8..115S . free.
- Foroughi . Ismael . Tenzer . Robert . Comparison of different methods for estimating the geoid-to-quasi-geoid separation . Geophysical Journal International . Oxford University Press (OUP) . 210 . 2 . 2017-05-19 . 0956-540X . 10.1093/gji/ggx221 . 1001–1020. free .
