Geocentric Datum of Australia - Background

Why a geocentric datum:

1. A geocentric datum for Australia will facilitate the building of an integrated national geographic data infrastructure.
   
   
2. Australia’s existing data infrastructure has not been produced on a geocentric datum and its datasets are not to national standards and specifications. Much of it is in graphical form and yet to be digitised.
   
   
3. 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.
   
   
4. 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:

1. 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;
     
     
2. Provides the basic common framework which facilitates interchange of data between various geographic information applications;
   
   
3. 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 comonplace in surveying and defence applications, it is being used more frequently for navigation and by many users requiring positional information.

GPS receivers will 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 wilh 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.

Defence
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.

Environmental management
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.

Recreation
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.

Aviation
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.

Legal Traceability

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.

Timing the change to a geocentric datum

ICSM has recommended that Australia adopt a geocentric datum by the year 2000, however, this move will be progressive from 1994.

Existing datums and coordinate systems will be supported by ICSM jurisdictions during the transition period until the GDA is fully implemented.

Map makers will employ the new datum on the basis of user demand.

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