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Surveying issues

Historical

Governor Darling recognised the importance of a proper system of survey for the colony 190 years ago and directed a trigonometrical (trig) survey to commence. This survey formed the basis of Sir Thomas Mitchell’s map of the colony in 1831 where he adopted the longitude at Parramatta Observatory and latitude determinations at Lake George, Warrawolong and other trig sites as a datum.

With the technical bits and pieces of spheroids hidden behind volumes of tables, a large portion of the surveying community in New South Wales remains unaware of the different coordinate systems and datums that were used since Mitchell’s time. Those involved with map making and geodesy have recollections of the Transverse Mercator (TM) grid system (in yards) using Clarke’s 1858 spheroid and that’s about all.

Recent history saw the Australian Geodetic Datum 1966 (AGD66) adopted for mapping purposes by the National Mapping Council in 1965, using Johnston Geodetic Station in the Northern Territory as the origin and the Australian Map Grid (AMG) as a projection.

In the early 1970s New South Wales adopted a system of survey integration, where the Integrated Survey Grid (ISG) was introduced to minimise projection corrections for cadastral surveys. It was determined that corrections for a 2° zone were less than 1:8000 and could therefore be ignored. AGD66 remained as the datum.

In 1984 the opportunity arose to change datums again, however New South Wales along with Victoria, ACT and Tasmania declined as the rest of Australia moved on to the Australian Geodetic Datum 1984 (AGD84). This decision was taken with the knowledge that yet another change, to a geocentric datum, may not be too far away.

November 1994 saw the adoption of GPS-derived coordinates for the Australian Fiducial Network (AFN), allowing Australia to position itself within a truly global mathematical framework and minimising the distortions introduced by deflection of the vertical, refraction and other phenomena in our physical world. These coordinates defined the Geocentric Datum of Australia 1994 (GDA94) using the reference ellipsoid GRS80 (Geocentric Reference System 1980).

Constrained by the geocentric datum, a national adjustment of geodetic observations was then undertaken and completed in June 1997. New South Wales’ contribution was some 40,000 terrestrial and GPS observations which produced a coordinate set known as GDA94 for over 3,000 sites in this State.

Since the GDA94 adjustment, several factors have highlighted the need to make improvements to GDA94 and the Australian Geospatial Reference System (AGRS). Australia’s physical position on the Earth has shifted north-east some 1.5 metres since 1994 due to tectonic motion while positioning accuracy has improved – all while uptake of and reliance on positioning and spatial information across industry and community applications has skyrocketed. Further, distortions in the original GDA94 adjustment of up to 0.3 metres are apparent in NSW.

In May 2015, in response to these issues, ICSM endorsed a “two-frame” datum modernisation strategy to support current and future positioning needs. The first of the two frames, the Geocentric Datum of Australia 2020 (GDA2020), is a new, static reference frame defined by 109 state-of-the-art GNSS reference stations across Australia, and propagated through the Australian State and Territory survey networks through a combined national least squares adjustment of approximately 2,000,000 measurements across 250,000 survey marks. Since 1 July 2019, New South Wales has delivered survey coordinates in GDA2020. 

The second of the two frames, the Australian Terrestrial Reference Frame (ATRF) is a time-dependent reference frame for high-precision applications only. ATRF is available through Geoscience Australia from 2020.

Survey Control

Following a GDA2020 state adjustment consisting of 30,000 control marks in NSW, GDA2020 coordinates for the majority of our 250,000 state survey control marks in SCIMS were generated using the grid transformation method (NTv2). These transformed values will be replaced on a region-by-region basis over the next few years by re-adjustment of the original observations.

Since July 2019, SCIMS Online provides coordinates, heights and metadata in both GDA94 and GDA2020, as well as AHD.

Projections

The projection for GDA is the Map Grid of Australia (MGA), i.e. MGA94 for GDA94 and MGA2020 for GDA2020. In both cases, this is a standard 6° Universal Transverse Mercator (UTM) projection, which is used by all states and territories across Australia.

All GDA projection coordinates issued by Spatial Services are MGA. While this does not preclude the use of other projection systems for specific projects, a system of survey integration must be maintained to ensure compatibility across all geographic systems at the local, national and global level.

The Surveyor-General of NSW has also endorsed the use of a ‘GDA Lambert’ projection for state-wide GIS and mapping applications. Refer to the Policy for NSW Lambert Conformal Conic Projection (PDF 3.6 MB) for more information.

Details of the parameters for all projections and ellipsoids relevant to NSW are listed on our Map Projections page.

MGA combined scale factor and orientation

The Combined Scale Factor (CSF) is applied to distances measured on the ground to reduce them to a map projection, or vice versa.

The GridCalc (GDA94) for NSW spreadsheet package includes CSF calculation for GDA94 and is available from ICSM’s GitHub repository (GridCalc_GDA94_NSW.xlsm will require Microsoft Excel 2007 or later).

Legislation

References to datums or coordinate systems are found in the following legislation:

Surveying and Spatial Information Act 2002 – see NSW Legislation website.

Surveying and Spatial Information Regulation 2017 – see NSW Legislation website.

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