Normal heights is a type of height

above sea level Height above mean sea level is a measure of the vertical distance (height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum. In geodesy, it is formalized as '' orthometric heights''. The ...

introduced by Mikhail Molodenskii. The normal height

H^*

(or

H^N

) of a point is computed as the ratio of a point's geopotential number (i.e. its geopotential difference with that of sea level), by the average, normal gravity computed along the

plumb line A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. It is a precursor to the spirit level and used to establish a verti ...

of the point. (More precisely, along the ellipsoidal normal, averaging over the height range from 0 — on the

reference ellipsoid An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximatio ...

— to

H^*

; the procedure is thus recursive.) Normal heights are thus dependent upon the reference ellipsoid chosen. The Soviet Union and many other Eastern European countries have chosen a height system based on normal heights, determined by geodetic precise levelling. Normal gravity values are easy to compute and "hypothesis-free", i.e., one does not have to know, as one would for computing orthometric heights, the density of the Earth's crust around the plumb line. The reference surface that normal heights are measured from is called the quasi-geoid (or quasigeoid), a representation of

mean sea level There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value ( magnitude and sign) of a given data set. For a data set, the '' ari ...

similar to the ''

geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is exten ...

'' and close to it, but lacking the physical interpretation of an equipotential surface. The '' geoid undulation''

N

with respect to the

: :

N=h-H

finds an analogue in the so-called height anomaly,

\zeta

: :

\zeta=h-H^*

The maximum geoid–quasigeoid separation (GQS),

N-\zeta

, is on the order of 5 meters in the

Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over ...

. Alternatives include orthometric heights (geoid-based) and dynamic heights.

References

Vertical position Geodesy {{Geodesy-stub

See also

References