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Surveying issues

Historical

Governor Darling recognised the importance of a proper system of survey for the colony over 170 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 'new' 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 1970's 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 and 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 define the Geocentric Datum of Australia (GDA) using the reference ellipsoid GRS80 (Geocentric Reference System 1980).

Constrained by the new datum, a national adjustment of geodetic observations was then undertaken and completed in June 1997. New South Wales contribution was some 40000 terrestrial and GPS observations which produced a coordinate set known as GDA94 for over 3000 sites in this State.

Survey Control

GDA94 coordinates for the majority of our 160000 state survey control marks were generated using the grid interpolation 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.

Adjusted GDA94 coordinates are available from SCIMS which now supports both datums for historical data, but new marks are maintained in GDA only.

There will be no change to survey practice, simply different numbers and the need to understand the issues during the transition period to the new datum.

Projections

The projection for GDA is the Map Grid of Australia (MGA). This is a standard 6° Universal Transverse Mercator (UTM) projection and is used by all states and territories across Australia.

All GDA projection coordinates issued by Spatial Services – Department of Finance, Services and Innovation are MGA. There is not an ISG 2° zone equivalent. This does not preclude the use of other projection systems for specific projects, however a system of survey integration must be maintained to ensure compatibility across all geographic systems at the local, national and global level.

There are a number of benefits in using MGA:

  • Less zone boundaries across NSW.
  • Many issues to do with crossing state borders will be resolved.
  • Large changes to the numerical values of coordinates, avoid confusion when using survey plans prepared on ISG.
  • MGA is a standard UTM projection used by all software applications.

The Surveyor General of NSW has also endorsed the use of a GDA Lambert projection for state-wide GIS and mapping applications. See the Lands Lambert Policy (PDF 3.6 MB) document for more information.

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

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.

The effect of changing from an ISG 2° zone to an MGA 6° zone will vary across NSW. The combined scale factor correction may be larger and there will be a change in orientation for ISG zones 1 and 3 of up to 1° 20'.

For example:

Town ISG Zone MGA Zone ISG
CSF
MGA
CSF
Orientation
ISG-MGA
Queanbeyan 55/3 55
0.999841
159ppm
0.999990
10ppm
1º 09' 31"
Bathurst 55/3 55 0.999871
129ppm
1.000210
210ppm
1º 06' 07"
Kellyville 56/1 56 0.999930
70ppm
1.000030
30ppm
-1º 06' 37"
Salamander Bay 56/2 56 1.000025
25ppm
0.999685
315ppm
Zero
Port Macquarie 56/2 56 0.999941
59ppm
0.999601
399ppm
Zero
  • Download a spreadsheet (XLS 19.5 KB) that will calculate the MGA Combined Scale Factor.
  • View a graph (nomogram) for the MGA Combined Scale Factor.

Legislation

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

  • Surveyors (Practice) Regulation 1996
  • Survey Coordination Regulation 1993
  • Mining Act 1992
  • Mining (General) Regulation 1992
  • Petroleum (onshore) Act 1991
  • Passenger Transport (taxicab services) Regulation 1995

Summary

For those in the surveying industry who understand coordinate systems, the impact of changing to a geocentric datum is minimal when compared to the advantages of creating one homogenous national datum. GDA provides direct compatibility with the Global Positioning System (GPS), minimises the need for transformations and allows our spatial data systems to move to a common 'user friendly' environment across Australia.

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