Sect_11.1.1

11.1.1 Transforming GPS Surveying Results: Coordinate Systems & Datums

INTRODUCTION

The results of a GPS survey, the minimally constrained 3-D network, are generally of limited use to the client as he/she usually requires the coordinates of points to be given in relation to a previously defined geodetic datum. This datum may be:

one previously established by GPS, or
a conventional "local" geodetic datum, usually comprising two distinct systems, one for HORIZONTAL position and the other for the HEIGHT component.

Both are essentially handled in the same way, that is, it cannot be assumed that the earlier GPS survey is on the same datum as the present survey, hence it cannot be integrated in the same manner as would individual GPS session solutions into a campaign solution, as described in Chapter 9. As the most common problem is to relate GPS survey results to prior terrestrial surveys, this chapter discusses the procedures for transforming GPS results between coordinate systems in order to facilitate the integration of the results into conventional survey datums. The procedures are, however, equally applicable to the combination of two GPS surveys, relating the GPS datum to other space datums, and even relating two terrestrial networks.

There are several distinct issues involved:

The MATHEMATICAL RELATIONS between the various commonly used coordinate systems: Cartesian, ellipsoidal, topocentric and projection.

Definition of the TRANSFORMATION between the 3-D coordinate systems involved: that implied by the GPS network on the one hand, and that defined by the local control stations on the other.

GPS HEIGHTING: as it relates to both the transformation process and the direct determination of elevation.

NETWORK VALIDATION and CONSTRAINT: involving the manipulation of the GPS network to make it consistent with the local datum through the incorporation of local control station information.

These topics are dealt with in this and the following chapter. GPS transformation can be carried out in two ways:

Using published transformation parameters.
Through the determination of the appropriate transformation parameters.

The transformation models are presented in this section, and the transformation options in Australia are given in section 11.2.6. Where the transformation parameters between the GPS datum and the local geodetic datum are to be determined, stations whose coordinates are known in the local geodetic datum must also be occupied during the GPS survey.

Back To Chapter 11 Contents / Next Topic