The deviation N between the ellipsoidal height h and the orthometric height H can be calculated by
2.
The reference surface for orthometric heights is the geoid, an equipotential surface approximating mean sea level.
3.
Then, the traditional connection between orthometric heights " H " and ellipsoidal heights " h ",
4.
Thus small corrections must be applied to field measurements to obtain either the dynamic height or the orthometric height usually used in engineering.
5.
Orthometric heights are usually used in the US for engineering work, although dynamic height may be chosen for large-scale hydrological purposes.
6.
Elevation may be specified as normal height ( above a reference ellipsoid ), orthometric height, or dynamic height which have slightly different definitions.
7.
So not showing the orthometric height of the sea ( and water bulges at the equator on the geoid sphere and lower at the poles ).
8.
Since gravity is not constant over large areas the orthometric height of a level surface is not constant, and NGS orthometric heights are corrected for that effect.
9.
Since gravity is not constant over large areas the orthometric height of a level surface is not constant, and NGS orthometric heights are corrected for that effect.
10.
The definition of NAVD 88 uses the Helmert orthometric height, which calculates the location of the geoid ( which approximates sea level ) from modeled local gravity.
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