Understanding your survey's elevation is crucial. This article explains the key differences between ellipsoid, geoid, and orthometric heights. Learn what geodetic height and geoid undulation mean and how they are used to determine accurate elevation (Mean Sea Level) for your Propeller site.
Explaining survey heights: Ellipsoids vs. geoids
You can use Propeller's free online coordinate converter to convert between coordinate reference systems.
Both ellipsoids and geoids are elevation measurements. They are theoretical representations of Earth's surface known as vertical datums.
Ellipsoids and Geoids: What's the difference?
- 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 that accounts for its irregular and undulating surface.
Geoid models are region-specific grids that allow conversions from the ellipsoid to national vertical datums. Propeller supports geoids from around the world.
How Do These Three Heights Relate to Each Other?
The elevation of a point of interest can be based on different heights:
- 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, so we usually need to transform it. An ellipsoid is a simplified representation of the terrain that doesn’t consider the Earth's actual shape.
- 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.
- Orthometric height (H) is the distance between the point on the Earth’s surface and the geoid. For practical purposes, it is the height above or below Mean Sea Level when extended to land areas. Surveyors or engineers who need accurate data usually use this height.
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