Ellipsoid height, geoid and orthometric height
Height above a geometric reference is not automatically height above a gravity reference.
Key knowledge
An ellipsoid is a geometric Earth reference; a geoid is a gravity equipotential reference approximating global mean sea level, distinct from local tides and the actual instantaneous sea surface. Ellipsoidal height and orthometric height therefore differ. For compatible references and a local parallel-normal approximation, ellipsoid height is approximately orthometric height plus geoid undulation.
Values and equations
h_ellipsoid approximately = H_orthometric + N_geoid
- h_ellipsoid
- height above the specified ellipsoid along its normal m
- H_orthometric
- orthometric height referred to the compatible gravity equipotential m
- N_geoid
- signed geoid undulation above the ellipsoid, positive outward m
Conditions and scope
- Declare the ellipsoid, gravity/vertical datum and geoid model, with the sign convention for undulation.
- The simple relation neglects normal/plumb-line geometric differences. Local mean-sea-level and tidal datums are not universally identical to the geoid.
- Compatible reference surfaces and sign conventions; local approximation neglecting nonparallel normal/plumb-line effects.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Inspected NOAA reference-system, geopotential/ellipsoidal/tidal-datum bodies and OS §§3.1.3–3.1.4, printed pp.13–16, including the parallel-line approximation. Uses its own explicit h/H symbols; OS uses the opposite capitalisation. No centimetre-accuracy or datum-date claim.
2026-10-10 · Independent Mathematics and Information editor (/root/core_math_information). Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.
Entry details
Permanent link to this entry ↗- Record identity
- foundation-earth-height-reference-surfaces
- Revision
- foundations-refresh.1
- Structured reference record
- Foundational reference additions