Orthometric height
Encyclopedia
The orthometric height is the distance H along a line of force from a given point P at the physical surface of an object to the geoid
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...

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Orthometric heights are what are usually used in the US for ordinary engineering work. Values for measured points can be obtained from the National Geodetic Survey data sheets . This data was gathered over many years by massive projects of precise optical leveling.

Orthometric heights relate approximately to the "height above sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

" but the current NAVD88 datum is tied to a specific elevation at a chosen point rather than to any location's exact mean sea level.

GPS measurements give earth-centered coordinates
ECEF
ECEF stands for Earth-Centered, Earth-Fixed, and is a Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system. It represents positions as an X, Y, and Z coordinate. The point is defined as the center of mass of the earth, hence the name Earth-Centered...

, usually displayed as height above the reference ellipsoid
Reference ellipsoid
In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body....

, which cannot be related accurately to orthometric height above the geoid unless accurate gravity data is available for that location. NGS is undertaking the GRAV-D program to obtain such data.

Alternatives to orthometric height include dynamic height
Dynamic height
Dynamic height is a way of specifying the height of a point above a reference, as opposed to orthometric height or normal height.Dynamic height is constant if one follows the same gravity potential as they move from place to place. Because of variations in gravity, surfaces having a constant...

 and normal height
Normal height
Normal heights are heights above sea level, one of several types of height which are all computed slightly differently. Alternatives are: orthometric heights and dynamic heights....

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The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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