Changes to coordinate system
Geocentric Datum of Australia
The Australian Geodetic Datum (AGD) was adopted by Australia in October 1966. The coordinates (latitude & longitude) produced by the 1966 national adjustment of terrestrial survey observations, in terms of the AGD are known as AGD66 and the derived UTM grid coordinates are known as AMG66 (ICSM, 2009, p. 6). In 1982 a new national adjustment, was performed using all of the data from 1966 adjustment, more recent survey observations and included a geoid model. This adjustment is known as Australian Geodetic Datum 1984 (AGD84), and its UTM grid coordinates are known as AMG84 (ICSM, 2009, p. 6). Some Australian jurisdictions adopted this new datum, while others such as NSW and ACT continued to use the earlier AGD66 (Manning & Harvey, 1992, p. 87).
In 1987, the Intergovernmental Committee on Surveying and Mapping (ICSM) initiated the conversion from the regional AGD66 and AGD84 coordinate datums to an Earth centred (geocentric) datum (Manning & Harvey, 1992; ICSM, 2009). ICSM resolved to adopt the Geocentric Datum of Australia and recommended the Australia-wide implementation of this datum by 1 January 2000. Geodetic coordinates on this system (latitude and longitude) are known as GDA94 coordinates and the Universal Transverse Mercator (UTM) grid coordinates are known as Map Grid of Australia 1994 (MGA94) (ICSM, 2009). The adoption of GDA94 will lead to a homogeneous national coordinate datum for surveying, mapping and GIS applications that is compatible with satellite-based coordinates (Manning & Harvey, 1994).
Since the adoption of the GDA, Global Navigational Satellite Systems (GNSS) have proliferated, and technological methods in positioning have become more reliable and faster.
Although the original GPS survery campaigns showed up discrepancies in the earlier AGD66 systems, the newer technologies are inturn showing problems in the GDA94 network. These are most noticeable at state borders, but also within the states themselves.
Consequent upon this will be the introduction of a news geodetic datum for Australia (GDA2015). This will include a "velocity model" to account for the Australian crustal plate's 7cm p.a. North-eastern drift. In the meantime, geodesists from the NSW government have refined the co-ordinates of the GDA94, and called them "GDA94(2010)". The original sets are referred to as "GDA94(1997)"
Both 1997 and 2010 definitions of GDA94 will therefore have to work in tandem for some years.
Essentially 1997 was tied to local marks based on AGD66, whilist 2010 is a direct connection to the Australian Regional GPS Network (ARGN) exclusively by GNSS observations.
To connect to local ground marks based on 1997 via GNSS surveying 2010 surveyors must perform a "site calibration" (or site localisation).
Essentially, this is a 7-parameter transformation from the national datum (GDA94(2010)) to local GDA94(1997) datum.
The reference below to an excellent article by Janssen E Mcelroy may be found at http://www.lpma.nsw.gov.au/surveying/corsnet-nsw/reference_material
In the ACT, this site calibration procedure is performed between GDA94(2010) to ACT Standard Grid Co-ordinates (SGC). It is essentially the same procedure a surveyor undertakes in NSW, but to the local ACT datum.
The ACT Government Survey Office is itself extending GDA94(2010) co-ordinates on ACT control marks, with the objective of creating a new local datum, intially to be based on the GDA94(2010). This will be a Transverse Mercator projection that will emulate the current system, but with very different co-ordinate "numbers".
This will make the new definition easily distinguishable from SGC (unlike that which exists between AMG, MGA94(1997) and MGA94(2010) but will be directly related to the latest realisation of the Geocentric Datum of Australia
Use of ACT Standard Grid Coordinates
The ACT Standard Grid is a Transverse Mercator map projection that uses the longitude of Mt Stromlo Trig Station as its central meridian. It is based on the AGD66 (Wellspring, 1973). Derivation of MGA co-ordinates for SGC marks by GNSS surveys is being made on a progressive, ongoing basis. However, ACT Standard Grid Coordinates continue to be the standard for all spatial data sets throughout the ACT for the foreseeable future.
A network of survey control marks has been established throughout the ACT with MGA94 coordinates surveyed to either A/1 or B/2 standards of accuracy (ICSM, 2007). This has allowed for the development of rigorous 7-figure transformation parameters to convert ACT grid coordinates and MGA94 coordinates and vice versa. As a result, all data within the ACT can be provided in any datum or projection with insignificant loss of accuracy.
Maps
Published 1:10,000 and 1:2500 series maps are available showing ACT grid, AMG66 and AGD66 geographical coordinates. In the future, these maps will be annotated to the effect that they are not on GDA94 and information will be available to show how equivalent GDA94 coordinates can be determined.References
Intergovernmental Committee on Surveying and Mapping. (2007). Standards and practices for control surveys (Version 1.7).
Intergovernmental Committee on Surveying and Mapping. (2009). Geocentric datum of Australia technical manual (Version 2.3, Amendment 1).
Janssen, V. and McElroy, s. (2010) coordinates and CORSET-NSW: Dealing with distortions in GDA94. Position, Dec/Jan 2011 24-27
Manning, J. and Harvey, W.M. (1992). A national geodetic fiducial network. The Australian Surveyor, 37(2), 87-90.
Manning, J. and Harvey, W.M. (1994). Status of the Australian geocentric datum. The Australian Surveyor, 39(1), 28-33.
Wellspring, K.H., (1973). Some aspects of the conversion from imperial to metric co-ordinates in the Australian Capital Territory. Technical Papers of the 16th Australian Survey Congress, Canberra.
