Background

New South Wales Land and Property Information (LPI) has established a network of Continuously Operating Reference Stations (CORS), known as CORSnet-NSW, which has greatly expanded the availability of high accuracy Global Navigation Satellite System (GNSS) measurements across the state. However, coordinates obtained using CORSnet-NSW are not consistent with the coordinates of survey marks contained in the Survey Control and Information Management System (SCIMS). This is due to the Geocentric Datum of Australia 1994 (GDA94), which is used to express SCIMS coordinates, suffering from distortions that would degrade the performance of CORSnet. As a stop-gap solution an ad-hoc realisation of GDA94, known as GDA94(2010) was developed to define the positions of CORSnet stations.

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CORSnet-NSW station locations and service areas as of October 2011 (click for full image).

The difference between SCIMS and CORSnet-derived coordinates, shown in the figure below, creates the need for a method of obtaining coordinates using CORSnet-NSW that are consistent with SCIMS, as this would deliver the high productivity of satellite positioning in tandem with the reliability and legal standing of SCIMS. Site Transformations are the method recommended by LPI to deliver SCIMS-consistent GNSS positioning for CORSnet-NSW users, and are the focus of this thesis.

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Distortions in GDA94 vs GDA94(2010) at selected CORS across NSW (click for full image)*.

What are site transformations?

Site transformations are used by surveyors in applications where GNSS techniques, such as NRTK, are desirable but need consistency with fixed marks. In order to perform a site transformation a set of transformation points (TPs) that surround the survey site and have known coordinates obtained from SCIMS, as well as from high accuracy GNSS, is selected. These TPs are then used to determine the transformation parameters between the coordinates of the selected points with respect to the GNSS observations in WGS84, and the SCIMS values in GDA94. The area within the boundary formed by the TPs for a given site transformation is known as the valid area.

This thesis will focus on a particular form of site transfromtion know as the seven parameter similarity transformation which is shown below in graphical form.

The 7 Parameter Similarity Transformation

A graphical representation of the seven parameter similarity transformation.*

*Image taken from Paul, B., & Toms, R. (2004, January 6). Spatial Reference Model: An Overview. Retrieved May 2, 2011, from SEDRIS Technologies: http://www.sedris.org/stc/2004/tu/srm/sld033.htm

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