Orthometric height

Orthometric height is the distance from a point on Earth’s surface to the geoid, measured along a curved plumb line. It represents elevation above mean sea level as defined by the geoid.

Geoid: An equipotential surface of Earth’s gravity field that approximates global mean sea level. Orthometric height ends at this gravity-defined reference surface.
Geopotential number: The difference in gravity potential between a point and a reference equipotential surface. It provides a gravity-based quantity from which orthometric height is derived.
GNSS: Satellite navigation systems that determine positions using signals from multiple orbiting satellites. GNSS gives ellipsoidal heights that require a geoid model to obtain orthometric heights.
Ellipsoidal height: The distance of a point from a reference ellipsoid, measured along its normal. It uses a smooth mathematical surface, not the geoid and curved plumb line.
Gravity: The attraction between masses, which near Earth shapes the direction of plumb lines and the geopotential field. Gravity determines the curved path along which orthometric height is measured.
Spirit leveling: A surveying method that transfers height differences using a level instrument and graduated staffs. Leveling supplies observed height differences that can be corrected for gravity.
Geoid model: A mathematical representation of the geoid relative to a reference ellipsoid. Surveyors use it to convert satellite-derived ellipsoidal heights into orthometric heights.
Quasi-geoid: A gravity-related reference surface used to define normal heights, especially where orthometric heights are inconvenient. It is the reference surface paired with normal height, an alternative to the geoid-based system.
Plumb line: The local direction of gravity, indicated by a freely suspended plumb bob. Orthometric height follows this direction rather than a straight vertical coordinate.
Dynamic height: A height-like quantity obtained by dividing a geopotential number by a fixed reference gravity value. Unlike orthometric height, it uses one conventional gravity value rather than mean gravity along a plumb line.

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