Geocentric Datum of Australia 2020 (GDA2020)
Find about Australia's national datum and how it has been implemented. Learn about map projections and transformation methods.
What is GDA2020?
The Geocentric Datum of Australia 2020 (GDA2020) is Australia’s national datum. It has replaced GDA94.
Some of the benefits of GDA2020 include:
- a higher accuracy than GDA94
- closer alignment with GPS and GNSS positioning services
- nationally supports consistent datasets, free of the known distortions of GDA94.
GDA2020 coordinates are approximately 1.8m to the north-east of GDA94 coordinates, which represents the tectonic motion of the Australian plate between 1994 and 2020.
GDA2020 is the first product of the modernised Australian Geospatial Reference System (AGRS), which provides the framework for coordinating all spatial information in Australia. Modernising the datum ensures that users have access to accurate and nationally consistent spatial data and services, and the associated economic benefits.
It is the legal datum for NSW, supported by amendments to the Surveying and Spatial Information Act 2002 and Regulation 2024 (S&SI Act and Regulation).
Australian Geospatial Reference System
The Australian Geospatial Reference System (ARGS) is the collection of reference frames, CORS infrastructure, geodetic models, and standards managed by the Intergovernmental Committee on Surveying and Mapping’s Geodesy Working Group.
It includes products such as the Australian Terrestrial Reference Frame (ATRF) and the Australian Vertical Working Surface (AVWS) which have been available to use since 1 January 2020.
Geographic Information Systems and web-mapping
We provide authoritative datasets in the Geocentric Datum of Australia (GDA2020). We also continue to support requests for data in alternate datums.
Geographic Information Systems (GIS) users should update software and workflows to support GDA2020 and/or associated transformations. Contact your software vendor for GDA2020 support.
GIS software supporting the NTv2 grid ‘conformal plus distortion’ transformation is recommended.
WGS84 and Web Mercator misalignment
World Geodetic System84 (WGS84) and its ‘Web Mercator projection’ have historically been used as a defacto standard in web mapping applications. Unfortunately, these applications often assume that GDA94, WGS84 and GDA2020 datum are all approximately equal, or within an acceptable margin of error.
- Misalignments of approximately 1.8m may occur when projecting GDA94 and GDA2020 spatial data into WGS84/Web Mercator. This issue affects all web mapping and GIS software.
WGS84 in the web mapping context is actually a low-accuracy static ‘ensemble’ of different WGS84 versions defined over time, and usually ignores the movement of the tectonic plates (up to 7cm per year in Australia).
Depending on the source, a WGS84 dataset may be equivalent to GDA94, GDA2020, ATRF at a specific date, or other regional datums. Historically, this information is rarely used to qualify WGS84 metadata. WGS84, without further clarification, is therefore suitable for low accuracy applications only (about 3m). Read more about WGS84 and misaligned spatial data in Australia.
Metre-level misalignments can occur when combining WGS84 datasets from different sources. Many WGS84 datasets would be better described in terms of well-defined datums of high accuracy where known (for example, GDA94 or GDA2020). You should ask for additional metadata to better define WGS84 data before collating datasets.
Surveying issues
Survey control
Initially GDA2020 coordinates for the majority of our 280,000 state survey control marks in the Survey Control Information Management System (SCIMS) were generated using the 'conformal and distortion' NTv2 grid transformation method. Currently more than 58% have been readjusted natively in GDA2020.
SCIMS Online provides coordinates, heights and metadata in both GDA94 and GDA2020, as well as AHD.
Projections
The projection for Geocentric Datum of Australia (GDA) is the Map Grid of Australia (MGA):
- MGA94 for GDA94
- MGA2020 for GDA2020.
In both cases, this is a standard 6° Universal Transverse Mercator (UTM) projection, which is used by all states and territories across Australia.
All GDA projected coordinates issued by NSW Government agency Spatial Services are MGA.
The Surveyor-General of NSW has also endorsed the use of a 'GDA Lambert' projection for state-wide GIS and mapping applications. See the NSW Lambert Conformal Conic Projection policy.
Details of the parameters for all projections and ellipsoids relevant to NSW are listed on Map projections.
MGA combined scale factor and orientation
The Combined Scale Factor (CSF) is applied to distances measured on the ground to reduce them to a map projection, or vice versa.
The GridCalc (GDA2020) for NSW spreadsheet package includes CSF calculation for GDA2020 and is available from Intergovernmental Committee on Surveying and Mapping’s GitHub repository.
History of surveying systems in NSW
NSW Governor Ralph Darling recognises the importance of a proper system of survey and directs a trigonometrical (trig) survey to commence. This forms the basis of Sir Thomas Mitchell’s 'Map of the Colony'.
Sir Thomas, who was the state's Surveyor-General, adopted the longitude at Parramatta Observatory and latitude determinations at Lake George, Warrawolong and other trig sites as a datum.
Australian Geodetic Datum 1966 (AGD66) is adopted for mapping purposes by the National Mapping Council, using Johnston Geodetic Station in the Northern Territory as the origin and the Australian Map Grid (AMG) as a projection.
NSW adopts a system of survey integration, where the Integrated Survey Grid (ISG) is introduced to minimise projection corrections for cadastral surveys. It is determined that corrections for a 2° zone were less than 1:8,000 and could therefore be ignored. AGD66 remains as the datum.
NSW – along with Victoria, ACT and Tasmania – sticks with AGD66 while the rest of Australia moves on to the Australian Geodetic Datum 1984 (AGD84). This decision is taken knowing another change, to a geocentric datum, may not be too far away.
Adoption of GPS-derived coordinates for the Australian Fiducial Network (AFN), allowing Australia to position itself within a truly global mathematical framework.
These coordinates define the Geocentric Datum of Australia 1994 (GDA94) using the reference ellipsoid GRS80 (Geocentric Reference System 1980).
A national adjustment of geodetic observations is undertaken. NSW’s contribution is about 40,000 terrestrial and GPS observations which produce a coordinate set known as GDA94 for over 3,000 sites in the state.
Improvements to GDA94 and the Australian Geospatial Reference System (AGRS) are needed due to:
- Australia’s physical position on the Earth shifting north-east approximately 1.5m since 1994 due to tectonic motion
- positioning accuracy improving
- distortions in the original GDA94 adjustment of up to 0.3m are apparent in NSW.
Intergovernmental Committee on Surveying and Mapping (ICSM) endorses a datum modernisation strategy to support current and future positioning needs.
On July 1, NSW delivers survey coordinates in Geocentric Datum of Australia 2020 (GDA2020).
GDA2020 is a new, static reference frame defined by 109 state-of-the-art GNSS reference stations across Australia. It is propagated through the country's survey networks through a combined national least squares adjustment of approximately 2 million measurements across 250,000 survey marks.
The Australian Terrestrial Reference Frame (ATRF) is available through Geoscience Australia. It is a time-dependent reference frame for high-precision applications only.