What Are Ellipsoid and Geoid Heights?

For a deeper dive into geoids and ellipsoids, check out our blog post.

Ellipsoid and geoid height concepts are crucial to understanding coordinate reference systems and how they are used in surveying and photogrammetry. 

You can use Propeller's free online coordinate converter if you need 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 which assumes the Earth is a squashed sphere that is "perfect" or completely smooth.

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. Propeller supports 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. This is why we usually need to make a transformation; an ellipsoid is a simplified representation of the terrain, which 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, is used to convert 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. This height is usually used by surveyors or engineers who need accurate data. 

I still can't do it! 

We wrote these articles to arm you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.

If you're stuck, the Propeller data success team may be able to help. You can contact our support team by clicking the support tab on the top-right pane of your user portal.