Statement on Datum Modernisation

24 March 2016

Executive Summary

This document overviews the technical detail of, and proposed implementation strategy for modernising Australia’s datum. Drivers for changing the present datum, the key elements of the modernised datum, and the Roadmap for its implementation are summarised. A two stage process which nominally concludes in 2023 is proposed.

In Stage–1 with user implementation commencing on 1 January 2017, a new and more rigorous national adjustment will be undertaken with the coordinates projected to a reference date of 1 January 2020 as a conventional plate-fixed or static datum. Horizontal coordinates will shift by approximately 1.8 metres, however the new datum will be more closely aligned to the globally standardised International Terrestrial Reference Frame (ITRF), such that residual coordinate differences will be small enough to be ignored for most users.

In Stage–2 a new national reference frame that will allow locations of points and their movement over time to be modelled, accommodating coordinates that can change over time, will commence implementation on 1 January 2020. This Earth-fixed reference frame will be highly accurate with respect to ITRF. Importantly, a conventional plate-fixed datum will also be retained in perpetuity, unless it becomes obvious that it is no longer needed.


The Geocentric Datum of Australia 1994 (GDA94) was officially adopted across Australia on 1 January 2000 and ICSM continued to promote its implementation until 2003. Between 2002 and 2003, ICSM prepared a discussion paper on the future of the datum where it was anticipated that GDA94 would ultimately be unable to satisfy the expectations of all positioning stakeholders as global positioning systems developed. The closing observation of this work was that a three dimensional readjustment may be necessary within 10 years. ICSM finally resolved that GDA94, as adopted in 2000, be retained for at least five years and then reassessed.

Drivers for Change

Since 2000, the geospatial community, like many areas in society, has experienced substantial technological change. These changes have been particularly profound, and driven mainly by the significant increase in understanding of the value of geospatial information among decision makers at many levels. It is clear that this rate of change is unlikely to decrease, but will increase.

Accordingly, since 2010, experts in the geospatial community have devoted significant resources to understanding how future trends and societal demands will impact on spatial data management, and what delivery tools will be required to service those demands. For example, see the United Nations GGIM Future trends in geospatial information management: the five to ten year vision and the Australian Strategic Plan for GNSS. These studies highlight a number of future trends that by 2020 will have significant consequences for the provision of a national datum.

The major trends identified by the PCG include:

  • The fully operational and enhanced global and regional satellite navigation constellations including Galileo (Europe), GPS (USA), GLONASS (Russia), Beidou (China), QZSS (Japan) and IRNSS (India) that will enable affordable consumer grade GNSS positioning to better than 50 cm accuracy and augmented positioning to better than 3 cm accuracy in real–time
  • A growing economic dependence on highly accurate positioning to underpin efficient transportation, spatially enabled business and government, and industrial automation (e.g., robotic mining and precision agriculture)
  • A growing reliance on spatially enabled business processes that in turn utilise machine to machine (M2M) communication of high–accuracy positions and high resolution geospatial data in real time and three dimensions
  • Globalisation of coordinate and geodetic frameworks as a result of increased global services, the development and widespread adoption of non–Australian consumer (non–enterprise) geospatial applications
  • Many users including those in the transportation sectors (aviation, maritime and road) operate within globalised frameworks that are constrained by requirements that limit the support of individual national datum and have a necessity to conform to international standards.

The reality of these trends underscores the fundamental and increasing requirement for close alignment of national datums with ITRF. These trends also highlight the inadequacies of GDA94, which has large coordinate distortions of up to 0.3 metres (predominantly present at jurisdictional borders), lacks rigorous coordinate uncertainty estimates, and will be offset from ITRF by 1.8 metres by 2020.

Datum Modernisation Roadmap 2011 — 2014

ICSM’s Permanent Committee on Geodesy (PCG) commenced an assessment of GDA94 in 2010 that, as predicted in 2003, identified that the national datum would soon be unable to serve the spatial requirements of Australian society. This subsequently led to the creation of the first version Datum Modernisation Roadmap.

The 2011 Roadmap proposed a two stage process to implement a new datum. In Stage–1 it was proposed to readjust the entire national geodetic network projected to reference epoch of the year 2020. In this adjustment, tectonic motion corrections would be applied and coordinates mapped forward in time. This new national adjustment would be repeated at regular intervals until Stage–2, when it was anticipated that the geospatial community would have ready access to the standards, tools and services that would enable a transition to a reference frame where the coordinates in the national datum are coincident with ITRF. The 2011 Roadmap anticipated that Stage–1 would be implemented in 2015 and Stage–2 in 2020.

Between 2012 and 2014, PCG undertook extensive promotion and communication of the key elements of the Roadmap within all Australian jurisdictions, and developed a good cross–sectional understanding of business and user requirements. These discussions served as a useful way to solicit feedback. The user engagement process has reaffirmed the view of ICSM in 2003 that GDA94 had a limited lifespan as a datum capable of serving the broader needs of the geospatial community. It has also given PCG confidence that the proposed changes are manageable and not prohibitively costly or risky for users to implement. In fact, the opportunity costs of not changing datum are regarded as particularly significant.

In 2013, ICSM agreed to support the development of the technical components of the Datum Modernisation Roadmap in conjunction with CRC for Spatial Information (CRCSI). At that time, ICSM requested PCG and CRCSI to jointly develop an implementation proposal for endorsement by both ICSM and ANZLIC in accordance with the Positioning Theme of ANZLIC’s Foundation Spatial Data Framework (FSDF). The following briefly summarises the outcomes of this work.

Characteristics of the Future Datum

Considering societal and geospatial trends, the associated user requirements of the datum, and learnings from ongoing technical endeavours, it has been identified that a future datum should:

Accuracy and Interoperability

  • Have an accurate and traceable connection to ITRF
  • Support positioning with horizontal coordinate uncertainties better than +/– 2 cm
  • Be fully three dimensional and tightly coupled with an improved national geoid model
  • Seamlessly integrate with the National Positioning Infrastructure (NPI)
  • Seamlessly integrate with future Satellite Based Augmentation System (SBAS) capabilities

Coordinate and Measurement Management

  • Support international best–practice approaches to coordinate uncertainty modelling
  • Enable time–based coordinate corrections including plate–motion and deformation models
  • Implement an approach to more sophisticated management of geodetic information based on input measurements rather than simply by output coordinates
  • Be continuously updated as new measurements are contributed and blunders detected

User Interface

  • Enable users to contribute measurements
  • Adopt an automated machine–to–machine approach for the exchange of geodetic measurements and information between each of the Australian jurisdictions, the private sector and public
  • Easily integrate with web–services to enable the delivery of geodetic products and services to the mass–market


  • Be fully supported by a range of technical and non–technical user resources provided in advance of formal datum adoption
  • Be widely implemented by spatial technology vendors before formal adoption in other FSDF themes.

Revision of the Roadmap

In late 2014, it became apparent that the 2011 Roadmap proposal to implement Stage–1 of the modernised datum sometime in 2015 would not be possible or desirable. This was primarily due to:

  • The delay in the release of the International Terrestrial Reference Frame 2013 (ITRF2013), now ITRF2014, which was seen as an integral element of a modernised datum
  • The complexity involved in preparing jurisdiction observational datasets and the resourcing constraints affecting this task in some jurisdictions
  • The feedback from the spatial sector which underlined the critical importance of having developed and provided the necessary tools, resources and education to datum users to enable them to support the adoption of a modernised or new datum well in advance of the datum roll–out
  • Recognition of the importance of widespread vendor adoption of the tools and resources.

PCG met in Canberra in February 2015 to reflect on these delays and revise the Roadmap. Following a careful and lengthy analysis of user feedback on the practical aspects of datum implementation and the current status of the technical implementation aspects, PCG agreed to a revised Roadmap for submission to ICSM and ANZLIC for endorsement.

Datum Modernisation Roadmap 2015

PCG’s revised Datum Modernisation Roadmap sets out an incremental implementation with two major stages. The Stage–1 implementation has been given a working title of Geocentric Datum of Australia 2020 (GDA2020) while Stage–2 has a working title of the Australian Terrestrial Reference Frame (ATRF). The use of working titles reflects that neither name has been finalised — this is a task to be undertaken by a yet to be formed datum implementation committee.

Stage–1 and Stage–2 of the modernisation will implement a two–frame concept in which both a conventional datum (GDA2020) and a reference frame (ATRF) will be simultaneously supported. From a user perspective, Stage–1 will emphasise a familiar static datum where coordinates are expressed with respect to a single reference date (1 January 2020). Stage–2 implementation, while providing continued support for GDA2020, will focus on providing support for users natively working within ATRF.

Three key coordinate modelling capabilities will be developed. The first of the capabilities will be a new National Plate Motion Model which will model the differences in coordinates that result from long–term crustal motion. The second will be a National Distortion Model which will model the differences in coordinates that result from differences in geodetic adjustments as new measurements are added and blunders detected. The final capability will be a National Deformation Model which will be a gridded national model of the differences in coordinates that result from crustal deformation, excluding long–term crustal motion which is defined in the National Plate–Motion model. Only Stage–2 will have additional deformation modelling capabilities to support high accuracy applications.

Stage–1 GDA2020

The GDA2020 datum will result from a readjustment of the entire national geodetic network to a reference epoch of 1 January 2020. This will correct regional decimetre–level biases remaining in GDA94, and also introduce a systematic horizontal shift due to plate tectonics of approximately 1.8 metres compared to the GDA94 coordinates of the same point. A reference epoch of 1 January 2020 has been selected because it is anticipated that by 2020 the vast majority of positioning will occur directly in ITRF at the epoch of observation.

Using international research on GNSS markets (refer: European Global Navigation Satellite Systems Agency, October 2013, GNSS Market Report Issue 3) the PCG has concluded that in the period 2012–2022 up to 96% of revenues generated by positioning are likely to be by user communities who will predominately rely on ITRF based positioning services. That is, the vast majority of users relying on the national datum to connect positioning and navigation services with mapping products at sub–metre accuracy will have access to the current ITRF position before the GDA94 position. The 1 January 2020 epoch choice will, until 2025, ensure that GDA2020 is consistent with ITRF to better than 50 cm and thus satisfy this majority of datum users without any specific treatment of kinematic coordinates.

The adjustment of the national geodetic network will be repeated regularly as new measurements are contributed and blunders detected. These readjustments would all be carried out using an epoch of 1 January 2020 so that coordinates associated with each update would reflect changes due to additional measurements and improvements to processing and adjustment strategies, and would not reflect crustal motion. In other words, coordinates would still be expressed in epoch 1 January 2020. For most users these coordinate changes of the geodetic infrastructure would be within the formal uncertainty estimates of the initial GDA2020 adjustment, and could therefore be ignored. Initially, these regular updates may not even be publically released, but will be used by the Australian jurisdictions to test and refine the systems and processes required to support the new datum.

Continuously Operating Reference Station (CORS) and survey control marks will be realised within the ATRF, which will be a dense national realisation of the most recent ITRF within Australia. A National Plate Motion Model will be produced to enable coordinates to be transformed between the ATRF and GDA2020. A national distortion model will be produced in parallel to all adjustments to enable users to transform coordinates between GDA94 and GDA2020.

It is anticipated that implementation of GDA2020 by users will be progressively achieved over a period of three years. It is not proposed to change any of the map projection details associated with the Map Grid of Australia (MGA) i.e. MGA2020 eastings, northings and zones would result from a UTM projection of GDA2020 latitudes and longitudes; just as MGA94 is a UTM projection of GDA94. Also, it is not proposed to change the Australian Height Datum as part of Stage–1.

Key elements of GDA2020 will include:

  • Regular readjustment of the national geodetic network at a reference epoch of 1 January 2020
  • Improved GNSS heighting through a refined national geoid model
  • Support for distortion modelling between GDA94, GDA2020 and ATRF
  • Support for national plate motion modelling between GDA94, GDA2020 and ATRF
  • For the first national adjustment and associated models to enable implementation testing in other FSDF themes, the implementation commencement date: 1 January 2017
  • Target adoption date: 1 January 2020

Stage–2 ATRF

From 2020, it is proposed to transition from a conventional datum to a reference frame. The national reference frame, the ATRF, will accommodate coordinate locations that change over time and be highly accurate with respect to ITRF. Importantly, GDA2020 (or its successor) would also be retained in perpetuity, unless it became obvious that it was no longer needed. Appropriate national plate motion, deformation and distortion models will be used to propagate coordinates between any desired epochs. Given the tools and services accompanying the new reference frame, the geospatial community will be afforded the flexibility to adopt a fixed reference epoch (whether by national convention or arbitrarily chosen on a project by project basis) without compromising data quality and data integration that would otherwise be inevitable with GDA94 or GDA2020. The ATRF will be an authoritative realisation of coordinates and velocities of CORS and survey control marks throughout Australia. Once implemented, the ATRF will provide the Australian community with a sustainable, traceable, high–precision geodetic reference system capable of meeting the most demanding positioning requirements.

Like Stage–1, it is anticipated that implementation will be progressively achieved over a period of three years between 2020 and 2023. It is not proposed to change the map projection details associated with the ATRF i.e. ATRF eastings, northings and zones would result from a UTM projection of ATRF latitudes and longitudes. The Australian Height Datum will be reviewed and stakeholder feedback sought as part of Stage–2.

Key elements of the ATRF will include:

  • Continuous readjustment of the entire national geodetic network
  • Support for GDA2020 for those users with a preference for a fixed reference epoch datum
  • Support for national plate motion modelling between GDA2020 and ATRF
  • Support for deformation modelling between GDA2020 and the ATRF
  • Support for distortion modelling between GDA2020 and the ATRF
  • Implementation commencement date: 1 January 2020 (estimated)
  • Target adoption date: 1 January 2023 (estimated).

Permanent Committee on Geodesy (PCG)
24 March 2016

Please direct feedback to: John Dawson, Chair PCG.