• Spatial information
• Background to GDA
• National implementation
• Implementation in Queensland
• NRW implementation

Spatial information

Spatial information is information associated with position on the surface of the earth (e.g. position of roads, property boundaries, soil types and human geography).

An important aspect of managing spatial information is providing a suitable system for describing position. Surveys, maps, charts and spatial data applications require a coordinate reference system called a geodetic datum so they can be located in their correct position on the earth's surface.

The currently used Geocentric Datum of Australia 1994 (GDA94) has been developed to optimise the use of new technologies in positioning and computing. It will provide the fundamental framework to support the Queensland Spatial Information Infrastructure Strategy (QSIIS). GDA94 replaces the previously used Australian Geodetic Datum 1984 (AGD84), as a more accurate system necessary for satellite positioning and computer information systems.

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Background to GDA

The coordinate system used in Australia has evolved through response to demands by business users and the availability of technology. The datum adopted in 1966 was refined in 1984 and then again in 1994 to establish the coordinate system presently used for all surveying, mapping and spatial information uses.

At the time it was adopted in 1966, the datum was based on the best available information and derived using the best available technology. However, when it was refined in 1984, it was recognised that Global Positioning Systems (GPS) and computer information systems were advancing at unprecedented rates and would eventually require Australia to adopt a geocentric datum.

The Inter-Governmental Committee on Surveying and Mapping (ICSM) addressed this issue in 1994. ICSM is responsible for supporting the development and implementation of national and international standards for surveying and mapping, including coordinate systems.

In accordance with this charter,'the meeting of the Inter-governmental Committee on Surveying and Mapping (ICSM) held in Canberra on 28-29 November 1994 adopted the following new geodetic datum for Australia and recommended its progressive implementation Australia-wide by 1 January 2000. Designation - the Geocentric Datum of Australia (GDA).' (Commonwealth of Australia Gazette (No. GN35, 6 September 1995).

Coordinates related to AGD84 will differ from those related to GDA94 by about 205 metres. GDA94 is identical with the datum used for Global Positioning System (GPS), ensuring that every advantage can be derived from satellite technologies.

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National implementation

Since the decision was made to adopt GDA94, ICSM has developed the necessary infrastructure to support the new geocentric datum, in collaboration with State organisations.

To implement GDA94, organisations had to ensure that systems presenting information in terms of position do so in relation to GDA94. This was not a simple task, and required careful planning. Each organisation needed to consider its business environment and decide on an appropriate implementation strategy. The strategy depended on the nature of their business, interaction with other organisations where spatial information is involved, technical competencies and the level of available support.

It is expected that any costs associated with the conversion to GDA94 will be borne by each organisation, with funding being obtained through existing means.

A wide range of support, particularly advice and technical information, is available from ICSM and its member organisations. This support addresses both business and technical issues.

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Implementation in Queensland

Since 1988, Queensland has fully supported National initiatives working towards the adoption of a geocentric datum. As the datum has evolved, so has Queensland's implementation strategy for the adoption of the Geocentric Datum of Australia 1994.

Adoption of the new datum will impact most Queenslanders to varying extents. The impact will vary for recreational users; for persons involved in working with spatial information; for very technical development of systems and applications; and for strategic managers and planners.

Because of this variation, it is crucial for everybody to assess your own circumstances and, if appropriate, develop an implementation strategy.

Implementation in Queensland commenced on 1 January 2000 when the Department of Natural Resources released key fundamental data sets referenced to the Geocentric Datum of Australia 1994 (GDA94).

The Digital Cadastral Database (DCDB) and the Survey Control Database (SCDB) are available on GDA94.

A wide range of support is available addressing both business and technical issues, however, it is expected that any costs associated with the adoption of GDA94 will be born by each organisation.

NRW implementation

This section contains details of the implementation strategy for each data set in NRW. Information will be added to this section progressively as individual strategies are determined.

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Topographic mapping (new publications)

Since 1 July 1998 all topographic maps have been produced on GDA94.

The GDA94 logo is displayed on the map cover and in the title block.

A note of the datum is included in the title block.

Topographic maps produced on GDA94 include data overlaps outside the standard map edges if they adjoin maps on AGD84 to compensate for gaps and slivers which occur between the two datums.

The inclusion of the overlap will be a consideration for each mapping project area, where in general only the maps on the eastern and northern extremities of the project would include an overlap if they adjoin AGD84 maps of a similar scale.

Maps contain the following definitions:

Datum: horizontal—Geocentric Datum of Australia (GDA 94); vertical—Australian Height Datum.

Grid: map grid of Australia, zone 56; central meridian 153°E at 1000 m intervals. Values are shown in full only at the south-west corner of the map.

Projection: map grid of Australia 1994, zone 56, which is a standard universal transverse mercator projection; central meridian 153° E.

Accuracy: horizontal—90% of well-defined detail is within ±12.5 m of true position; vertical—90% of elevations are within ± 2.5 m. These accuracies may not be achieved within areas of dense vegetation.

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Cadastral mapping

The DCDB was converted to GDA94 in 2000 and all cadastral mapping is now on this datum.

Tourist/thematic mapping

The digital thematic database from which recreational and tourist maps are generated is based on various projections and grids depending on the source material. In the Queensland series where scales vary from 1:500 000 to 1:2 500 000, the projection is Simple Conic and the shift to GDA94 would be negligible. These maps when revised will indicate GDA94 compatibility.

Regional maps at scale 1:90 000 to 1:250 000 are on AMG84 (AGD84) and show geographical coordinates and will be converted to GDA94 as part of map revision.

Town maps at scales 1:20 000 to 1:30 000 are stored on AMG84 (AGD84) and will migrate to GDA94 as part of map revision.

Contacts

For all technical, business and implementation related queries, please email Geodetic Support within Land Information Systems.

Links to GDA sites

• GDA (ICSM Site) - a wide coverage of business and technical issues.
• ICSM - providing links to all ICSM activities inculding ASDI, Topographic Data, Cadastral Data, Place Names, and of course Geodesy
• GDA Technical Manual (ICSM Site) - includes general introductory information, GDA specifications, etc)
• Geodesy (Geoscience Australia Site) - provides links to many associated topics, including general papers on datums and projections; hierachy of geodetic networks and GPS.