Adjustment of the Combined State and
Territory Geodetic Networks

The GDA coordinates of the Australian Fiducial Network (AFN) of permanently tracking GPS sites, and the Australian National Network (ANN) of existing GPS coordinated survey marks, were determined by the processing of global GPS observations.

The next stage of the GDA implementation required the existing geodetic survey networks to be adjusted to the framework provided by the AFN and ANN.  Initially it was envisaged that each State or Territory would carry out this task for their region, in their own timeframe.  However, it rapidly became apparent that a number of the States and Territories were prepared to do this in a similar timeframe.  The ICSM Geodesy Group therefore proposed that as many of the State and territory networks as possible be adjusted onto the GDA together.  Initially this project involved just a few ICSM jurisdictions and the project was colloquially referred to as 'the spine' adjustment.  Other jurisdictions subsequently joined the project so that this adjustment now includes all regions except Western Australia.

This 'spine' adjustment is an ICSM cooperative effort with each jurisdiction supplying and validating the data for their area.  The dataset consists of:

• The AFN and ANN GDA94 fixed positions
• All conventional observations (directions, distances etc) used in the AGD66 and AGD84 national adjustments, as well as additional conventional observations since 1984
• State GPS networks, typically with a spacing of about 100 km and an accuracy of the order of 1 part per million.

This results in a data set of more than 70,000 observations and over 7,000 stations.

Dr J.S. Allman, who carried out the AGD84 national adjustment, was contracted by ICSM for this project and, as for the 1984 adjustment, the NEWGAN software and Section method are used.  For this purpose the data was divided into twelve sections.

Although the process is a mathematically rigorous 3 dimensional adjustment, only the horizontal positions are to be adopted from the adjustment.  The Australian Height Datum (AHD) is not affected.

The results of this adjustment will be used by the State and Territory authorities to constrain their supplementary networks, as a continuation of the GDA implementation.

Western Australia has completed the observation of their GPS network and in the near future will adjust it to the AFN, ANN and the current 'spine' positions.

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