Technical Tips
Planning conversion to GDA94
by Ben Johnson, Senior GIS Consultant |
Spatial coordinates in textual databases
Spatially referenced imagery
Coordinate vector data
Conversion is not just data
Further information
The
Geocentric Datum of Australia (GDA) was officially in Australia
on the 1st January 2000. The implementation of GDA aligns Australia's
spatial coordinates with international systems and provides a
uniform coordinate system for the collection, storage and dissemination
of spatial information.
GDA
replaces the Australian Geodetic Datum (AGD) which has been in
place since 1966. The AGD provided a reference system that best
fitted the shape of the earth in the Australian Region but its
origin did not coincide with the centre of mass of the earth.
National datums were commonly non-geocentric before satellite
based navigation systems were established in the early 1970s.
Figure 1: The GDA strategy document produced by Spatial Vision
The
distance between the origin points of GDA and AGD is approximately 200
metres. When the coordinates of a point on the Earth's surface are converted
from AGD to GDA this translates to a coordinate difference of approximately
the same amount. The difference varies slightly depending on where you
are in Australia.
The change applies to all spatial data, including latitudes and longitudes,
and grid coordinates of northings and eastings.
Figure 2: GDA Shift (Source: ICSM website, 2003)
New South Wales and South Australia both passed legislation in 2000 to signify
the official adoption of GDA as the new datum, whereas Victoria took
a non legislative path. In practise, whilst many public and private
sector organisations have adopted GDA already, many others are still
planning their conversion.
The impact of postponing the adoption of GDA will increasingly become apparent
as major data sources move to GDA. Uniform adoption of GDA will also
reduce the potential for confusion as GPS, GIS and navigation systems
become more widely used in business and recreational activities.
One of the critical steps to planning conversion to GDA is to conduct an
audit of existing data and its sources held by the organisation to determine
whether data needs to converted or updated in GDA by a supplier. Spatial
related data may come in various forms, including vector, image and
coordinates. The conversion of each form will need to be considered.
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Spatial coordinates in textual databases
Spatial coordinates, typically in Australian Map Grid (AMG) held in databases
and business management systems, will require conversion to Map Grid
of Australia (MGA). MGA is a grid coordinate system based on the Universal
Transverse Mercator projection and GDA94.
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Spatially referenced imagery
The format and volume of spatially referenced imagery such as aerial photography
can be a significant issue when considering GDA conversion. Many organisations
have converted source photography files to alternative formats (with
better compression) for use in their corporate GIS systems. For example,
in TIFF format to ECW, MrSID or JPEG format. In many of these cases,
the conversion of the source image files is easier in technical terms
than the conversion of the files in a compressed format. However, access
to, and the shear volume of, source TIFF images can be an issue. All
image types can be converted, but the cost of the options need to be
evaluated on a case by case basis. Many organisations, find that the
most cost-effective approach is to request their image supplier to perform
the conversion.
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Coordinate vector data
The accuracy of transformed vector data is dependent on the accuracy of the
transformation method used. There are 3 available methods of datum transformation
to consider when transferring vector coordinates between AGD66 and GDA94:
- National grid Transformation: The National Transformation
grid national grid, which is in the Canadian NTv2 format and has an
accuracy of a few cm in dense urban areas.
- 3-Dimensional Similarity Transformation: Also known as 7
parameter transformation this transformation has an uncertainty of the
order of a 1 metre.
- Molodensky Transformation: Also known as 3 parameter transformation
and can be up to 10m accuracy
In Victoria, Land Victoria strongly recommends that the NTv2 National Transformation
Grid is used in the process of converting spatial information to GDA.
This approach will ensure that the conversion to GDA is as accurate
as possible and conforms to the conversion processes Land Victoria is
applying to the base (1:25,000 scale) datasets for the State, including
cadastre and planning scheme information.
There a range of software tools available to undertake data transformation.
The ICSM website provides a list of software vendors and their level
of compliance with GDA94 at the following URL: http://www.icsm.gov.au/icsm/gda/software.htm.
A decision on the most cost-effective approach to data conversion for
each organisation will depend on the nature and extent of spatial data,
and skill resources available. Some GIS software will have NTv2 compliant
transformation capabilities out-of-the-box. In other situations, organisations
will need to either purchase additional software to perform the data
translation or contract out the conversion.
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Conversion is not just data
In addition to digital conversion of data to GDA, organisations undertaking
the transition also need to develop and implement strategies and timetables
to:
- raise awareness within the organisation;
- discuss the changes with customers, data providers and users,
and industry bodies;
- clarify the timelines for the change, based on operational requirements,
and those of customers and providers;
- implement change, including training staff, converting spatial
data and maps;
- understand and integrate with data suppliers’ strategies;
and
- tell dependent organisations what is planned.
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Further Information
Further
information about GDA can be found on the Intergovernmental Committee
on Surveying and Mapping web site (www.icsm.gov.au).
Spatial Vision has been contracted by several organisations to develop impact
studies for GDA and road maps to plan migration, including the Municipal
Association of Victoria (MAV) and the City of Greater Geelong. For further
information about how Spatial Vision may assist you, please contact:
Ben Johnson on (03) 9691 3000
email: ben.johnson@spatialvision.com.au
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Copyright © Spatial Vision, Thursday, 29-July-2004
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