GPS Altitude

The native coordinate system used by GPS to represent a geographic position is a perfect ellipsoid defined by the geodetic datum WGS84. This ellipsoid tries to approximate the shape of the globe as good as possible all over the world.

However, the real shape of the globe is not an ideal ellipsoid, so the true sea level differs from the WGS84 sea level depending on your location (latitude and longitude) on earth.

There are huge data tables available e.g. on the internet providing parameters to correct the WGS84 altitude to the altitude above true sea level, depending on the geographic longitude and latitude. However, Meinberg GPS receivers don't use this since it doesn't affect the accuracy of timing.

Also, maps may be based on a different geodetic datum than WGS84, so the geographic coordinates and altitude values may refer to a different shape of an ellipsoid, and thus the coordinates may have to be transformed from one geodetic datum to another one before they can be compared.