An option to select between different altitude datums has been added to the RT software in the latest release. For automotive applications the datum is not normally important but many surveying customers choose to use our RT products and the altitude datum is critical for them.
In the options page of RT-Config the new altitude option is used to change the altitude datum between geoidal and ellipsoidal. A constant offset (from the ellipsoidal altitude) can also be entered. The default datum is the geoid and this is the datum that was used by the RT until this option became available.
What do these altitude datum options mean? Because the shape of the earth is not a perfect ellipse, it is not easy to agree on a global value of zero height. Different countries define ellipses that fit their local territory better than the WGS84 (or ITRF05). These are the datums used by GPS.
To complicate matters, mean sea level varies from WGS84 by 50m in places. GPS receivers used for marine applications normally use a look-up table to correct GPS measurements. They output zero at mean sea level. Surveyors prefer to have altitude output relatively to the ellipse because it is better defined.
Two terms are defined to make it easier to understand what products do.
- Ellipsoid – this is an ellipse that best fits the earth (or a portion of the earth). The RT always used WGS84 (now ITRF05) ellipsoid, which is the same as the normal ellipsoid of GPS.
- Geoid – this is the surface that defines constant gravity. It is the surface that mean sea level follows. On land the surface passes through the point that has the same acceleration (due to gravity, etc.) as mean sea level.
Other terms exist to convey the same information.
There are many different look-up tables to convert from the ellipsoidal altitude to the geoidal altitude. There is no mathematical formula. In the RT3000 and RT4000 products the EGM96 table is used.
