The orthometric height (symbol ''H'') is the vertical distance 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 direction as a reference line, or plumb-line. It is a precursor to the spirit level and used to est ...

from a point of interest to a reference surface known as 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 exte ...

'', the vertical datum that approximates

mean sea level A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...

. Orthometric height is one of the scientific formalizations of a layman's " height above sea level", along with other types of heights in Geodesy. In the US, the current NAVD88 datum is tied to a defined

elevation The elevation of a geographic location (geography), ''location'' is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational equipotenti ...

at one point rather than to any location's exact mean sea level. Orthometric heights are usually used in the US for engineering work, although

dynamic height Dynamic height (symbol H^\text or H^\text) is a way of specifying the vertical position of a point above a vertical datum; it is an alternative for orthometric height or normal height. It can be computed (in SI units of metre) by dividing the loca ...

may be chosen for large-scale hydrological purposes. Heights for measured points are shown on National Geodetic Survey data sheets, data that was gathered over many decades by precise spirit leveling over thousands of miles. Alternatives to orthometric height include

and

normal height Normal heights (symbol H^* or H^N; 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. it ...

, and various countries may choose to operate with those definitions instead of orthometric. They may also adopt slightly different but similar definitions for their reference surface. Since gravity is not constant over large areas the orthometric height of a level surface (equipotential) other than the reference surface is not constant, and orthometric heights need to be corrected for that effect. For example, gravity is 0.1% stronger in the northern United States than in the southern, so a level surface that has an orthometric height of 1000 meters in one place will be 1001 meters high in other places. In fact, dynamic height is the most appropriate height measure when working with the level of water over a large geographic area. Orthometric heights may be obtained from differential leveling height differences by correcting for gravity variations. Practical applications must use a model rather than measurements to calculate the change in gravitational potential versus depth in the earth, since the geoid is below most of the land surface (e.g., the ''Helmert orthometric heights'' of NAVD88). GPS measurements give earth-centered coordinates, usually displayed as

ellipsoidal height Geodetic coordinates are a type of curvilinear orthogonal coordinate system used in geodesy based on a '' reference ellipsoid''. They include geodetic latitude (north/south) , ''longitude'' (east/west) , and ellipsoidal height (also known as g ...

''h'' above 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 approximation ...

. It can be related to orthometric height ''H'' above the geoid by subtraction of the geoid height ''N'': :

H=h-N

The geoid determination requires accurate gravity data for that location; in the US, the NGS has undertaken the GRAV-D ten-year program to obtain such data with a goal of releasing a new geoid model as part of the Datum of 2022.

References

Surveying Geodesy Vertical position {{Geodesy-stub

See also

References