## Earth Geodetic Model

The **geoid** is the surface of the sea, continued at mean sea level on land.

In comparison to the WGS84 ellipsoid, which is an exact ellipsoid, the geoid is only “ellipsoid-shaped”. It has various undulations caused by gravitational anomalies of the earth.

The deviations of a digital elevation model from the true reference surface are compensated by a **vertical datum**. So the geoid is a particular kind of a vertical datum.

The gravitation on earth is not exactly the same. There exist small variations for places on earth which have a bigger landmass or density of the earth’s molten core. Those areas attract the water body of the sea so that the water forms a hill over the anomaly. The geoid describes the height variation of those hills or undulations compared to the WGS84 ellipsoid as shown in the following image:

For example, in Sardinia the geoid is about 50 meter higher than the WGS84 ellipsoid. For that reason you get an altitude of about 50 meters when using a GPS device on one of Sardinia’s beaches, when you would naturally expect 0 meter altitude.

For more information on the geoid see here:

To calculate the geoid height deviation we can use the following online calculator:

The calculator uses the so called **EGM** (Earth Geodetic Model) to calculate the geoid. The EGM is derived from satellite measurements of the earth’s gravity. There exist several versions of the EGM:

- EGM84 (30 arcmin resolution)
- EGM96 (15 arcmin resolution)
- EGM08 (1 arcmin and 2.5 arcmin resolution)

You can download the latest EGM08 DEM data set from here:

- http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_wgs84.html
- http://schorsch.efi.fh-nuernberg.de/data/terrain/EGM

More information on the above DEM is given on the EGM08 project page.