Sect_11.3.2

11.3.2 GPS Heighting

THE ROLE OF GEOID HEIGHT INFORMATION

Geoid height information plays two crucial roles in GPS result transformation:

To convert the external (non-GPS) control information into the mathematically equivalent reference system to which the GPS results refer. The control information is generally in the form of f, l (horizontal components) and H (the orthometric height), and the necessary steps are:

compute the ellipsoidal height (h = H + N), and
then the , , h system can be converted into Cartesian components (section 11.1.6).

The transformation parameters may then be determined between the two Cartesian systems, given at least three stations with coordinates in both the GPS system, and the local geodetic system in which the results are required.

The GPS-derived, and transformed (to the local geodetic system), 3-D Cartesian results can then be converted to the , , h system. However, to obtain orthometric height information the geoid height needs to be subtracted. This is referred to as GPS Levelling.

There are several comments that need to be made at this stage:

The geoid height information is required to be given in terms of the ellipsoid implicit in the local geodetic datum: the reference ellipsoid. That is, it must be non-geocentric.
The geoid height accuracy requirements vary considerably. For the transformation they are only modest, but for GPS levelling it depends on the orthometric height accuracy sought (usually similar to levelling standards -- section 10.2.4).
For survey accuracy GPS heighting, relative geoid height between ends of GPS baselines is required, as illustrated in the Figure in section 11.3.1.

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