12.1.1 Constraining GPS Networks

INTRODUCTION

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The results of a GPS survey are a set of 3-D coordinates, nominally in the WGS84 datum. Rarely are these results in the form that is useful to the client or sponsor of the survey. The results must therefore be converted into more "useful" quantities. This could require for example:

Separation of the results into horizontal and vertical components (the client may only be interested in the horizontal components).

Transformation to a local geodetic datum -- section 11.1.3 and section 11.2.1.

Reduction of the GPS heights to the same system as used for geodetic levelling -- section 11.3.1.

A further adjustment, or constraint, of the GPS network to "fit" into a previously defined network.

This list is not intended to be complete, or to imply that all of the above will necessarily be required. It merely serves to illustrate the relatively complex post-GPS adjustment operations that may need to be performed in order to make the GPS network results truly useful. Many of these operations require that external control information be used.

By "external information" is meant the position information of some GPS points (either 3-D coordinates, horizontal components only, or even just levelled heights), that have been obtained from:

• other GPS surveys, or
• conventional surveys, or
• official geodetic databanks.

There are a number of "external" adjustments that are pertinent to GPS surveying. Although the most common is of the GPS / local control adjustment, it is by no means the only type. For example, GPS surveys may involve "primary" points and "secondary" points. A greater effort may have been invested in coordinating the primary stations, with a higher percentage of baseline redundancies, longer observation sessions, etc. One adjustment strategy would be to carry out a minimally constrained multi-session adjustment for the primary stations, as described in section 9.3.1. To subsequently incorporate the session solutions for the secondary stations into a multi-session campaign solution in which the primary stations are held fixed.

However, to illustrate some of the steps and issues involved in combining external coordinate information with current GPS results the more common example of the integration of a GPS survey on the one hand, and local geodetic control information on the other will be considered.

The introduction of external geodetic control information into an already adjusted (though "minimally constrained") GPS network generally requires that:

• The VCV information of the GPS-only network should correctly reflect the true errors of the multi-session adjustment (section 9.4.1).
• Geodetic datum coordinates of the local control stations (occupied during the GPS survey) are somehow introduced into the GPS-only adjustment.
• The combined adjustment must account for the differences in the datums, through some transformation modelling (section 11.2.1).
• The appropriate weights between the different observations (GPS relative station coordinates on the one hand, and the externally defined coordinates of the control stations on the other), and the degree of constraint to be introduced into the combined GPS / external data adjustment, will need to be carefully specified.
• The combined adjustment should take into account the incomplete height information available concerning control stations. Usually only orthometric height is available, however the required ellipsoidal height contains geoid height information as well (section 11.3.1).
• As with any network adjustment, an examination of the combined network solution should be made to ensure that there are no inconsistencies which may indicate a problem with the GPS survey or the control station information. This is an issue of Quality Control.

The final "external" adjustment will usually (though not always) result in the 3-D coordinates of all the GPS stations, on the local geodetic datum, together with their uncertainties; as well as the local transformation parameters relating the GPS survey datum to the local geodetic datum.

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© Chris Rizos, SNAP-UNSW, 1999