Sect_11.3.7

11.3.7 GPS Heighting

GEOMETRIC OR INTERPOLATION METHOD

This method relies on the direct evaluation of N from eqn (11.3-1), where h is provided by the GPS survey and H is determined from conventional levelling. If there are a number of points in the network for which N can be estimated in this way, N at other points can be obtained from an interpolation technique such as plane surface (and other surface) fitting, collocation, cubic splines, etc.

The following are some characteristics, advantages and disadvantages of this method:

Simple to use, not requiring sophisticated software. No outside assistance is required and hence the GPS surveyor is in complete control of the computations.
Geoid height information can be obtained relative to a local (non-geocentric) ellipsoid.
Suitable where there are sufficient stations having both orthometric and GPS heights, and at the appropriate density to model the form of the geoid.
Because no assumption is made about the datum for either system of heights, it is particularly suitable where the local height system may not be rigorously defined. For example, because the Australian Height Datum is not a true orthometric system, this method is still applicable as the "geoid" height is not determined by independent means, but from the (possibly flawed) data itself.
The geoid height results can be represented as a contour map, or a set of coefficients for the functional model used for the interpolating surface.
Variable accuracy, dependent upon:
- quality and density of common station coverage, and
- smoothness of geoid (for example, linear interpolation over distances even as low as 25km can cause 10cm errors in parts of Australia).
Principal problem is the requirement to (GPS) survey levelled benchmarks (that is, additional stations). At least three are required if using the plane surface fitting option. Hence if GPS heighting is required, this aspect of additional fieldwork has to be borne in mind during the GPS survey planning process.
Further, the results cannot be used beyond the perimeter of the GPS network.

Summary: Variable accuracy, but simple to use, and hence a commonly applied technique for medium accuracy applications.

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