What Are Ellipsoid and Geoid Heights?

Ellipsoid and geoid height concepts are crucial to understanding coordinate reference systems and their use in surveying and photogrammetry. 

You can use Propeller's free online coordinate converter to convert between coordinate reference systems

Ellipsoids and Geoids: What's the difference?

Both ellipsoids and geoids are elevation measurements. They are theoretical representations of Earth's surface, called vertical datums.

An ellipsoid is a mathematical model of the Earth's surface that assumes it is a squashed, completely smooth sphere.

A geoid is a more accurate geometric representation of the Earth, which takes into account the irregularity and undulation of the surface. Geoid models are region-specific grids that allow conversions from the ellipsoid to national vertical datums. Propellersupports various geoids from all over the world. 

Three Different Heights

The elevation of a point of interest can be based on different heights.

1. Ellipsoid height (h) is the difference between the ellipsoid and a point on the Earth's surface. It is also called the geodetic height. 

When a GPS receiver measures elevation data, it is referenced to the ellipsoid, which is why we usually need to transform it. An ellipsoid is a simplified representation of the terrain that doesn’t consider the Earth's actual shape.

2. Geoid height/Undulation (N) is the offset value between the geoid and the ellipsoid models measured locally. 

This constant number, which can be either positive or negative, converts between ellipsoidal height and the local vertical datum height. 

3. Orthometric height (H) is the distance between the point on the Earth’s surface and the geoid. 

For practical purposes, it is the height below or above the Mean Sea Level when extended on land areas. Surveyors or engineers who need accurate data usually use this height. 


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