Geocentric Datum Of Australia (GDA) - History and Background
ICSM Decision Process
Up until the implementation of the Geocentric Datum of Australia (GDA), the coordinates being used over mainland and offshore Australia included: Pre-1966 (Clarke), AGD66, AGD84, WGS72 and WGS84. However, most spatial data in Australia was referenced to the Australian National Spheroid (ANS), which was oriented by one of the two versions of the Australian Geodetic Datum (AGD66 or AGD84). This combination of spheroid and datum were chosen to provide optimal referencing conditions for spatial data across the Australian continent. The ANS is, however, a rather poor reference system over other parts of the world.
The continuing development of global communication, transport and information technologies made the emergence of a global spatial data infrastructure inevitable. By introducing a geocentric datum, Australia not only moved towards its own national spatial data infrastructure, but also closer to the global infrastructure. This ensured compatibility across various geographic information systems at the local, regional, national and global level. There was International support to move to a geocentric datum. Many countries in Europe, North America and the Asia-Pacific region has already developed national spatial data infrastructures within a global context. Australia’s move to a geocentric datum enabled full participation in that initiative.
In the lead up to GDA94, a geocentric spheroid and datum called the World Geodetic System (WGS84), had gained worldwide support for referencing spatial data, and especially GPS measurements. In Australia, a new spheroidal system was thus developed, employing the geocentric GRS80 spheroid, which at the time was, for all practical purposes, identical to WGS84.
GDA94 is a static coordinate datum based on the International Terrestrial Reference Frame 1992 (ITRF92), held at the reference epoch of 1 January 1994. At this time GDA94 and ITRF were coincident with WGS84 – the datum used for GPS. However, over time, both ITRF and WGS84 realisations have diverged from GDA94, primarily due to tectonic motion of the Australian plate (approximately 70 mm per year in a NNE direction) and improvements to ITRF. The absolute difference between GDA94 and ITRF is now approximately one metre. Since the WGS84 reference frame is now aligned to the ITRF2000 at the centimetre level, the absolute difference between WGS84 and GDA94 is also approximately one metre.
In 1988 the Intergovernmental Committee on Surveying and Mapping (ICSM) decided that Australia would move to the earth-centred coordinate system, GDA. This coordinate system is based on the International Terrestrial Reference Frame (ITRF) recommended by the International Association of Geodesy (IAG). The GDA is based on the Australian Fiducial Network (AFN) which fits into the ITRF. The AFN was used as the foundation to determine geocentric coordinates for the Australian National Network (ANN). The AFN and ANN are the basis of a unique national geocentric coordinate system, which, with regional networks, provide the fundamental framework for all geographic information over Australia.
The implementation of GDA Australia wide was a positive step towards consistency of datum between the many data themes. This important step also had economic and practical benefits for Australia. With a geocentric datum as Australia’s geodetic reference system, an integrated national geographic data infrastructure could be established. This national resource supports economic, social and environmental decision-making. In 1990, the International Federation of Surveyors recommended that its members promote and support the adoption of a global geocentric reference system proposed by the International Association of Geodesy. In 1991, the 56 member maritime nations of the International Hydrographic Organisation (IHO), recommended the production of future navigational charts on a geocentric datum. In May 1994 at the UN Regional Cartographic Conference for Asia and the Pacific, all member countries resolved to adopt a geocentric datum as soon as possible. On 6 September 1995, the new geocentric datum was Gazetted. The International Civil Aviation Authority (ICAO) decreed that WGS84 will be used for its activities, commencing 1 January 1998.
As Australia was an active IHO member, the Navy’s Hydrographic Service produced navigation charts and digital products on a geocentric datum. The Australian Department of Defence also produced geographic information on a compatible datum. The use of a geocentric datum facilitated reliable joint operations between allied forces such as the USA, the UK, New Zealand and Canada.
The responsibility to coordinate the implementation of GDA lies with ICSM. This Committee comprises Commonwealth, State and Territory agencies in Australia and New Zealand responsible for surveying and mapping to meet civilian and military needs.
Why a geocentric datum
- A geocentric datum for Australia facilitates the building of an integrated national geographic data infrastructure.
- Australia’s existing data infrastructure had not been produced on a geocentric datum and its datasets were not to national standards and specifications. Much of the data was in graphical form and yet to be digitised.
- The continuing development of communication and information software and hardware technologies, provides the mechanism for storage, rapid movement, and access of massive amounts of digital data.
- As a result of these advances in electronic accessibility, quick turn-around times from data acquisition to output of digital or graphical products are achievable using data from one source or a variety of compatible sources
Benefits of a Geocentric Datum
- Provides compatibility with GPS measurements allowing for the direct input of GPS field data into:
- datasets from other GPS surveys
- mapping datasets and geographic information systems produced on a geocentric datum;
- Provides the basic common framework which facilitates interchange of data between various geographic information applications;
- Eliminates the need for GPS users to have an understanding of datums and transformations; and, results in more efficient utilisation of resources and reduces overheads.
Changes will occur
There is a shift, or displacement, of approximately 200 metres across Australia in a north-east direction, between coordinates of points on the existing Australian Geodetic Datum (AGD) compared with coordinates of the same points on the Geocentric Datum of Australia (GDA).
Conversion to the GDA will be most noticeable on mapping products. With the geocentric datum, the map projection and mapping grid zones will remain the same. The borders of maps will have the same latitude and longitude but will be in slightly different positions on the ground. This means detail on existing maps cannot be joined with corresponding detail on maps under the new system. The impact of this ‘displacement’ will be greater on large scale maps (eg 1:5,000) compared with small scale maps (eg 1:100,000). The ‘displacement’ across Australia will be up to 210 metres.
During the transition to a fully operational geocentric datum, map makers may choose to print overlaps on the northern and eastern edges of maps produced on a geocentric datum to enable them to be joined to maps on existing datums.
GPS applications will grow
While the use of GPS is already commonplace in surveying and defence applications, it is being used more frequently for navigation and by many users requiring positional information.
GPS receivers have become as widely used as mobile phones and calculators, affecting our daily lives in ways not yet envisaged. Already small GPS receivers are being installed in motor vehicles for navigation, and simple hand held units can be obtained at moderate prices for a variety of applications.
Transport and Communications
GPS is a 24 hours a day, all-weather system, that can be used with satellite based communications to provide an invaluable aid to navigation and in times of distress. The transport industry will rely on GPS. Truck fleets, taxi and courier companies, and commercial shipping, will use GPS to monitor fleet movements. Safety and emergency services and law enforcement agencies will be major beneficiaries of GPS. They will use GPS for navigation to get to a location easily and quickly.
The operations of the UN Coalition Forces in the Gulf War during 1990/91 provided substantial evidence of the difficulties experienced in the field during military operations when coping with a variety of geodetic reference systems. It highlighted the necessity of having one reference system on a geocentric datum, compatible with GPS.
The need for a single coordinate system is paramount to avoid the possibility of errors which occur when using more than one system. A uniform series of maps and digital geographic data will facilitate rapid data display and data exchange, critical factors in times of crisis.
Geographic information is critical to improve the understanding and management of Australia’s natural resources and to protect the environment. Environmental management uses Geographic Information Systems (GIS) which are entirely dependent on position information. GPS provides much more accurate and reliable field position information than locating positions on traditional paper topographic maps. Better quality field measurements means that other information, such as vegetation types sourced from remotely sensed imagery, can be fully integrated into the GIS and verified.
In the outback, 4WD vehicles can easily have a GPS unit installed telling the occupants where they are located at any point in time. The same applies to cross-country balloonists, skiers, orienteers, bushwalkers, school groups, yachting or deep-sea fishing enthusiasts, or to any land, sea or air-based recreational activity.
Flights operating outside radar and VHF coverage are presently monitored on the basis of air traffic control clearances and against flight plans which describe the assigned route the aircraft is expected to follow. This provides limitations to efficient navigation and to safety aspects of monitoring aircraft movements. Recognising these limitations, the International Civil Aviation Organisation (ICAO) is moving to satellite navigation as a world aviation standard. Australia’s Civil Aviation Authority (CAA) already has approval for supplemental (as opposed to sole means) en-route navigation using GPS, and has commenced trials prior to its implementation.
These developments will lead to a more dynamic airspace, reduce aircraft congestion, and provide a safer and more efficient industry.
When a position determined from GPS is used to define a legal boundary (e.g. cadastral surveying) or is involved in a legal dispute (e.g. a maritime accident), it may be necessary to trace the measurement to a standard acceptable to the legal authorities. By continuously monitoring GPS data at a number of sites and comparing results, it is possible to provide assurance for GPS positions, to a national standard.