The Geoid model contributes to the vertical component of the reference system so that ellipsoidal GPS heights can be converted to orthometric elevations for  practical uses.

The real challenge lies in knowing the relationship between the ellipsoid and the geoid. Once we determine the difference between these two surfaces, called the "geoid-ellipsoid separation" or "geoidal height", at a given point, we can then apply the geoidal height to our GPS height measurement to get the mean sea level elevation.

The Geoid model

There are three key elements to consider when examining the geoid:

• Topography - The surface of the Earth
• Ellipsoid - GPS heights are referenced to this mathematical surface
• Geoid - The natural surface extension of mean sea level