Frequently Asked Questions on Australia’s Datum Modernisation




The Frequently asked Questions (FAQs) help to explain the technical aspects of datums, why modernisation of Australia’s datum is important and how this modernisation process may occur. Click on the questions below to go to the answer.

In the surveying world a datum is a reference point, set of points or surface, that provides known locations from which the locations of other features are determined. For example, a reference point could be the inlet level of a stormwater main, a set of points could be two corner pegs defining a property title boundary, while a common example of a reference surface is the floor of a building.
A geodetic datum is a reference surface, a coordinate system and a set of defined reference points from which coordinates may be calculated to define a specific location on the Earth’s surface, at a national or global scale. Geodetic datums are the fundamental positioning system for any country.
Location or positioning information has always been important for the mapping and surveying community but a technologically advanced society has become reliant on increasingly precise location information at a national scale in a vast array of areas including: business (construction, agriculture, insurance), safety services (police, emergency services, landscape mapping), modernising our transport and tourism industry (navigation via Global Navigation Satellite System (GNSS) enabled maps), automated vehicles, telecommunications and entertainment.
Australia has separate national horizontal and vertical reference systems. The current Australian geodetic datum, used to represent the horizontal location of features, is the Geocentric Datum of Australia 1994 (GDA94). The Australian Height Datum (AHD) is the current national vertical datum.

Nothing on the surface of the Earth is fixed. The Earth’s tectonic plates are constantly moving - in different directions and at different speeds – and deforming. Australia sits on one of the fastest tectonic plates, moving at about 7cm per year, or approximately 1.6 metres since 1994. Whilst this may appear subtle it is highly significant when it comes to the use of equipment that relies on accurate positioning data (linked to satellite navigation), e.g. devices such as mobile phones and in-vehicle navigation.

Regional scale deformation of the Australian landmass has been measured at the ‘several decimetre level’, requiring consideration of how society responds to issues such as sea-level rise, groundwater extraction and flood mitigation.
Commencing in January 2000, GDA94 was adopted as the national geodetic datum in response to a desire for the locations of mapped features to align with the locations relative to the WGS84 geocentric reference frame used by GPS receivers. The accuracy of most GPS devices at the time was to approximately 10 metres.

The advent of multiple GNSS constellations and continual improvement in GNSS technology will deliver the potential for all Australians to measure locations with accuracies of a few decimetres, and eventually a few centimetres. Modernising of the national geodetic datum will ensure it is directly compatible with these GNSS measurements, which will differ by approximately 1.8m to GDA94 coordinates by 2020. This gap will continue to increase.

The main advantage of GDA2020 is that the coordinates will be immediately compatible, at the decimetre level, with GNSS derived coordinates referenced to global coordinate systems utilised by GNSS, such as WGS84 or ITRF2014, and with other coordinate systems used in other countries.

It will ensure Australians continue to have access to the most accurate location-based information routinely achievable and hence, a more efficient and confident exchange of spatial information between digital systems, meeting the requirements of the digital age.

Datum Modernisation is required due to the significant growth in the number of users of precise positioning and the expected improvement in positioning technology over the coming decade.

The two–stage approach and implementation timeframe reflects a desire to manage the implementation of appropriate changes in advance of the anticipated need. This approach will accommodate what is a significant change in spatial data management that will require adequate lead time to implement; it will take many years to develop the modernised datums and promulgate them through public and private spatial datasets.

Making the first step to GDA2020 demonstrates that change is possible using current technology and spatial data management techniques. GDA2020 coordinates will be immediately compatible, at the decimetre level, with GNSS derived coordinates referenced to global coordinate systems utilised by GNSS such as WGS84 or ITRF2014, and will remain so for several years.

The adoption of the ATRF however will partly depend upon the development of international standards and the subsequent applications of these standards by software and hardware providers. For example, spatial referencing which incorporates the date of measurement as part of the coordinate reference system. Standards organisations and some international software providers are already considering this issue.

The choice of 2020 as the epoch of the national datum — effectively where Australia will be located in 2020 — will ensure that the datum has the best alignment to GNSS reference frames within the period 2017 to 2023. It is within this period that mass–market users are expected to obtain decimetre, or better, positioning capabilities referenced to global GNSS reference frames. Expert predictions over a number of years have consistently indicated 2020 as the date when this is likely to eventuate. Defining the location of the Australian national geodetic datum to 2020 should enable the majority of users and geospatial data to be managed in a consistent way.

Anyone that uses a mobile device or relies on location information will benefit from this work. From applications in building and construction, agriculture, landscape mapping, transport, insurance, emergency services, telecommunications — and more. All these sectors use location information that relies on the national geodetic datum. They will all benefit from access to information that is more closely aligned with commonly used global coordinate reference systems.

All members of the spatial information community will be required to make some changes to common practices to facilitate this change. This includes those people who take measurements on the ground, surveyors and mappers, those who manage and deliver spatial information, distributors of spatial measurement equipment and those who are responsible for updating geographic information software. Widespread availability of accurate positioning — a new spatial location paradigm — would necessitate this community to undertake changes, regardless of any action taken to modernise the national geodetic datum.

There will be some effort required by business, industry and government to update their systems and practices to reflect the new GDA2020 datum. However, changes would have been required to systems and practices of these entities to accommodate the emergence of accurate GNSS positioning irrespective of the datum modernisation initiative.

The development of a modernised two frame spatial reference system aims to provide a framework that limits the potentially disruptive effects arising from the widespread availability of accurate positioning information. ANZLIC and ICSM consider the immediate effort and cost in adopting GDA2020 to be outweighed by the advantages afforded the Australian community of a system that encourages the seamless exchange of digital spatial data in support of a spatially enabled society.

Yes. GDA2020 and the ATRF will be defined in terms of, and consistent with, the latest realisation of the globally standardised International Terrestrial Reference Frame (ITRF) and the Asia Pacific Reference Frame (APREF). New Zealand is also improving its geodetic datum to better meet user needs. The two countries have worked jointly on relevant aspects of their respective datum modernisation programmes.

The Intergovernmental Committee on Surveying and Mapping (ICSM) is responsible for modernising Australia’s datum.  In September 2015, the ICSM formed a dedicated working group to oversee the implementation of the modernisation of the Geocentric Datum of Australia.  The GDA Modernisation Implementation Working Group (GMIWG) has government representatives from the Commonwealth, States and Territories who have a broad range of knowledge and experience in the surveying and spatial sciences industry.  The group will consult with users and build the tools and technical resources needed to assist with the datum transition.  It is working to ensure that the practical implementation of the datum occurs seamlessly and with minimal disruption to existing systems and processes.

ICSM has indicated implementation of GDA2020 will commence in January 2017.  This means that transformation products and tools (GDA94 ⇔ GDA2020), supporting documentation and updated general educational resources on national datums, will be available by January 2017 via the ICSM and relevant jurisdiction websites.

There is currently no single date proposed for the adoption of GDA2020 by jurisdictions.  However, with GDA2020 becoming available from January 2017, the Commonwealth, States and Territories will have access to the information needed to begin transition to GDA2020 from that time.  Jurisdictions will be undertaking extensive consultation during 2016 with a view to making a decision on when they will adopt GDA2020.  Issues to be considered in the timing of the adoption date will include when survey control coordinates, for both traditional ground control and CORS, will be readily publically available and when the majority of Australian users of the national datum and interacting with accurate datasets, positioning and navigation tools will have the capacity to easily perform GDA94 ⇔ GDA2020 transformations.

Some work which is controlled by regulation, such as surveys or spatial data capture under Government contract, will be obliged to be completed based on GDA2020. Changes to legislation, regulations, policies and contracts that enforce this requirement will be implemented after January 2017, and the timeframe for these changes will be informed by consultation with relevant stakeholders and communicated well in advance of their occurrence.  Transition conditions will apply in some circumstances, and where appropriate guidelines will be developed to inform stakeholders of suggested practices.

It is envisaged that the adoption of GDA2020 as the operational datum will be primarily driven by practical considerations in relation to the changes in spatial technology and business requirements for use of this accurate location information. As government organisations and others change to GDA2020 it will be increasingly inefficient to convert inputs and outputs between GDA94 and GDA2020.

Jurisdictions are working together under the ICSM banner to develop the common resources and tools that support the technical definition and delivery of GDA2020 and the ATRF.  As with GDA94, there will be no cost to access this information, but some jurisdictions may ultimately charge for coordinate information provided from survey control databases for specific ground control marks, similar to the arrangements for GDA94.

Organisations and individuals will be responsible for implementing the necessary changes to systems and practices to support the adoption of new datums. The main costs are expected to relate to software ⁄ database upgrades, education and training.  Widespread availability of accurate positioning –a new spatial location paradigm - would have necessitated organisations and individuals undertaking training and software upgrade programs regardless of the modernisation of the national geodetic datum.

The ICSM Permanent Committee on Geodesy (PCG) undertook an assessment of GDA94 in 2010, which led the PCG to develop the first version datum roadmap in 2011. Since 2012, the PCG and researchers in the Cooperative Research Centre for Spatial Information (CRCSI), under Project 1.02 Next Generation Datum, have worked on the technical elements of datum modernisation, undertaken a user analysis and conducted a variety of workshops and forums.

GMIWG was formed in September 2015 to specifically address the practical aspects associated with datum modernisation, including determination of the date of adoption of GDA2020 by Australian jurisdictions and development and delivery of the resources and information supporting the technical tools. ANZLIC, ICSM, PCG, GMIWG and individual jurisdictions will continue to develop the resources required to support datum modernisation until its nominal conclusion in 2023.

The majority of the cost to jurisdictions associated with the datum modernisation effort is being funded from normal operational budgets in relevant government agencies.  Similarly ICSM has directed some of its general operation budget to fund aspects specifically associated with the datum modernisation initiative since 2012.

The CRCSI received specific funding for investigation of the geodetic aspects associated with development of the modernised Australian and New Zealand datums under its Positioning Program.

Questions relating to the nature of changes required and the potential impacts

At a technical level: All members of the spatial information community, including those who take measurements on the ground, surveyors and mappers, those who manage and deliver spatial information, distributors of spatial measurement equipment and those who are responsible for developing and updating geographic information software.  Members of this community will need to be clear about what processes are required to transform between different datums.  For those that are involved in the measurement and exchange of spatial information they must ensure they know what datum spatial information has been captured on, and where appropriate, deliver and store spatial information with appropriate metadata to ensure that this knowledge is retained with the data.

At a general community level:  As for many of the services available to us most members of the community who will use, and thus benefit, from GDA2020 will probably not even be aware the update has occurred.

In a sense it’s a little like when we turn on a tap and clean water flows, we know it comes from a water storage somewhere but are not likely to be aware of the extent of the river or reservoir catchment, the treatment and testing processes followed or the complex engineering challenges that enable this seamless ‘source to tap’ service.

The outcome from the change to GDA2020 for everyone is access to the most accurate location information achievable when using their GNSS enabled devices, such as mobile phones.

Official transformation parameters and tools to convert between GDA94 and GDA2020 and vice-versa will be developed by ICSM, led by Geoscience Australia in conjunction with other jurisdictions and the Cooperative Research Centre–Spatial Information.

The following products will be available:

  • A 2D transformation and distortion grid file in the widely used Canadian National Transformations version 2 (NTv2) format
  • 7 parameter (Helmer) transformation
  • A 3D transformation grid file — format yet to be determined.

Values supporting conversion of datasets between GDA2020 and ITRF2014 utilising either a plate motion model or 14 parameter transformation will also be published.

This information will be provided directly to the EPSG Geodetic Parameter Registry which is referred to by spatial software and hardware providers worldwide before incorporating transformation parameters into software and firmware.

The major software developers and their Australian distributors will also be contacted directly by PCG and GMIWG representatives to alert them to the release of the defining parameters of the new Australian geocentric datum.

These measures are being taken with a view to fast–tracking the adoption of GDA2020 into new releases of spatial software and hardware.

Members of the spatial information community should monitor the ICSM website and industry publications for updates on progress towards the adoption of GDA2020, and look out for opportunities for briefings on datum modernisation at industry events.

The most important action all individuals can take to prepare for datum modernisation is to ensure they are 100% certain of the datum of any spatial data they use.  This can happen immediately.

Along with ensuring knowledge of the datum forms part of every spatial data transaction, users can also begin to prepare for two other inevitable consequences of the widespread availability of accurate positioning — the requirement to record the date (epoch) of measurements and an estimate of its reliability.  This is generally referred to as the accuracy of the measurement, although in Australia the term “Positional Uncertainty” has been defined for identifying the rigorous determination of location measurement quality — see ICSM Special Publication 1 (SP1) for further information.

At a whole–of–project level we anticipate the ICSM website will be a key source of information about the project. It will host a range of resources such as FAQs, updates on current progress, technical descriptions and instructions, implementation guidelines or worked examples, links to more general education resources on modern national datums and explanations of the process and the benefits of updating to GDA2020 and then the dual frame system.

All materials are developed with the explicit aim of being a guide and may be further refined (as needed) to meet more specific jurisdictional level communication needs.

Importantly a special working group established by the ICSM, the GDA Modernisation Implementation Working Group (GMIWG) has formal responsibility for modernising or updating GDA94. Comprised of spatial experts from state, territory and commonwealth governments, the GMIWG is consulting with users and building the tools and technical resources needed to assist with the datum transition.  It is working to ensure that the practical implementation of the datum occurs seamlessly and with minimal disruption to existing systems and processes.

GDA2020 will replace GDA94 as the Recognised Value Standard (RVS) of position within Australia (see question “how is GDA94 defined” below).  It is understood it will take some time for the nation to transition to GDA2020 - projects commenced prior to January 2017 on GDA94, along with the need to revise legislation, regulations, specifications, policies and administrative arrangements that refer to GDA94 will ensure that locations expressed on GDA94 datum will appear long after January 2017.

For those requiring legal traceability to RVS of position and needing⁄wanting do work in GDA94, you can transform GDA2020 coordinates back to GDA94 and claim traceability to the RVS of position. This is a way for CORS providers to move to GDA2020 and continue to support GDA94 users in a manner compliant with the RVS of position.

GDA2020, or subsequent plate–fixed datum, will continue to operate in conjunction with the ATRF — that is, the national spatial reference system will be a dual frame system where the choice of GDA2020 or ATRF is determined by user requirements.  Explanations and education of how the two systems will operate in parallel will be progressively provided as resources are developed.

A GNSS unit displays and exports location information according to parameters defined in its relevant setting menus.  These settings are brand and model specific and it is up to the user of the device to confirm those settings prior to using or exchanging location information. 

It is generally possible to export information in formats that contain the datum information as metadata, but one of the current common generic formats used for exchanging spatial information — Comma Separated Value (CSV) files — typically does not include information about datum, or measurement quality, unless specifically requested by the user.

It is currently “industry best practice” for metadata to be included with spatial data that includes information about the datum on which it was measured.  If there is no metadata available, the datum may be able to be determined by independent field verification.  The data provider may also be directly requested to confirm the datum but information obtained in this manner should be used with caution.

No.  Paper maps are generally described as large–scale or small–scale. A large scale map depicts a smaller area in greater detail than a small scale map.  Common large scale map products in Australia are hard-copy street directories, which are typically at a scale of 1:10 000.  This means 1 millimetre when scaled on the hard–copy represents 10 000 millimetres on the ground, or 10 metres.  Hence a change in coordinates of 1.8 metres (from GDA94 to GDA2020) would equate to a displacement on the paper map of 0.18 millimetres, which is essentially undetectable.  The impact on small scale paper maps, which typically have scales smaller than 1:1 000 000 (or 1 millimetre on the map equals 1 kilometre on the ground), is effectively insignificant. 

All Australian CORS network Value Added Re–sellers (VARs) will ultimately deliver location services on GDA2020.  The exact arrangements for the move from GDA94 to GDA2020 by the CORS VARS will be determined by agreement between jurisdictions and the VARS, taking into consideration the views of CORS network clients and the overall timeframe for adoption of GDA2020.  The arrangements regarding the move from GDA94 to GDA2020 for CORS network positioning products will be publicised well in advance of the move to GDA2020, by the VARs and effected jurisdictions.

Questions addressing more technical detail and definitions

The International Federation of Surveyors (FIG) Reference Frames in Practice Manual contains more information about many of the topics in this section.

A three dimensional, Earth–centred Cartesian coordinate system is defined by the alignment of three mutually perpendicular axes; the Z-axis – aligned to the Earth’s mean axis of rotation; the X-axis – aligned perpendicular to the Z-axis and the line joining the centre of the Earth to the zero meridian; and the Y-axis – aligned mutually perpendicular to the X and Z axes.  The intersection of these three axes defines the Earth’s geocentre.

An ellipsoid is a three dimensional figure symmetrical about each of three perpendicular axes, whose plane sections normal to one axis are circles and all the other plane sections are ellipses.  An ellipsoid is used in geodesy to approximate the size and shape of the Earth.  It is defined by the radius of a circle aligned to the Earth’s equator, known as the semi-major axis, a, and a circle aligned to the north and south poles, known as the semi–minor axis, b.  The mathematical relationship of a to b is known as the flattening, f. More information about ellipsoids can be found here.

In the mapping or surveying world, a geodetic datum is a reference surface, a coordinate system and a set of defined reference points from which coordinates may be calculated to describe a specific location on the Earth’s surface. 

Australian geodetic datums include an ellipsoid and reference points whose three dimensional coordinates are described by latitude, longitude and ellipsoidal height which have been determined using the most accurate measurement systems available in the era they were established. 

A geocentric datum is where the origin of a geodetic datum ellipsoid is defined to be at the centre of mass of the Earth.  Examples include GDA94 and GDA2020.

The International Terrestrial Reference Frame, or ITRF, is a practical realisation of the International Terrestrial Reference System (ITRS) and is the fundamental standard global reference framework.

The ITRF coordinate system is a three dimensional, earth–centred Cartesian coordinate system. Effectively the axes appear “fixed” to the centre of the Earth, and are defined so that they appear to co–rotate with Earth in its motion in space.  As the tectonic plates move with respect to this “earth-fixed” framework the coordinates of reference points on the tectonic plates are observed to change over time.

This excellent article explains how ITRF realisations are developed, allowing very accurately determined “known” locations of points and their movement over time to be modelled and act as a framework for accurate time dependent location at a global level.  

ITRF is utilised by all GNSS systems (see also question “What is WGS84” below).  There have been nine realisations since 1992 and the current realisation, released in January 2016, is ITRF2014.

The “earth-fixed” ITRF is typically not regarded as a “datum” – rather it is the international standard reference framework to which national geocentric datums are aligned.

World Geodetic System 1984 (WGS84) is the reference frame used by GPS and developed by the U.S. Department of Defense (DoD).  It is now defined and maintained by the U.S. National Geospatial Intelligence Agency (NGA).  WGS84 has been revised five times since its original realisation in 1987.  

NGA and its predecessors have ensured that WGS 84 is consistent with the most recent ITRF realisation and have advised that the current realisation has been aligned at the centimetre level to the ITRF to support interoperability with other GNSS. 

The NGA Standards document NGA.STND.0036_1.0.0_WGS84 contains a full description of the DoD World Geodetic System 1984 and a summary of its relationship to other geodetic datums.

A national geodetic datum may be defined by reference points that are said to be “fixed” to one of the Earth’s tectonic plates.  The reference points move along with the tectonic plate and the coordinates appear to be unchanging with time. This is sometimes called a “plate-fixed” datum or alternatively a “static” datum (as the coordinates appear to be “static”).

As an alternative to a “plate-fixed” datum, a national geodetic datum may be defined like the ITRF so that its axes appear to co-rotate with Earth in its motion in space and are “fixed” to the whole solid Earth, rather than a tectonic plate.  So like ITRF the coordinates of reference points on a tectonic plate would be observed to change over time.

This is now starting to be referred to as an “Earth-fixed” datum but previously was often referred to as a “dynamic datum”.  Conceptually, the term “dynamic datum” confuses many people, who assume it to mean that the datum itself is constantly changing.  This is obviously not the case – the datum is uniquely defined and fixed in orientation and location.  Rather, an “Earth-fixed” datum allows the changes in coordinates of points on the Earth’s “dynamic” surface to be referenced and represented.

The Geocentric Datum of Australia 1994 (GDA94) is the current official geodetic datum for Australia endorsed by the Intergovernmental Committee on Surveying and Mapping (ICSM) which commenced implementation in January 2000.

The Geocentric Datum of Australia 1994 (GDA94) is defined by the coordinates realised on the Australian Fiducial Network (AFN) geodetic reference stations, referred to as the Geodetic Reference System 1980 ellipsoid (GRS80), determined within the International Terrestrial Reference Frame 1992, at the 1 January 1994 epoch.

The legal description of the datum is provided through a gazettal process as a Recognised Value Standard (RVS) under the National Measurement Act 1961. 

Access to GDA94 is via spatial connection to the AFN reference stations directly via linked GNSS measurements, or from other GNSS CORS stations with Regulation 13 Certification issued by Geoscience Australia, or via a traceable connection to geodetic survey control stations constrained to the AFN.

Since all these stations move along with the tectonic plate it is a so-called “plate-fixed” datum. 

The Geocentric Datum of Australia 2020 (GDA2020) is a national geodetic datum for Australia that is planned to be officially established in January 2017.  It will be defined by GDA2020 coordinates on a selection of Australian geodetic reference stations referred to the International Terrestrial Reference Frame 2014, at the 1 January 2020 epoch, using the Geodetic Reference System 1980 ellipsoid (GRS80). 

The legal description of the datum will be provided through a gazettal process as a Recognised Value Standard under the National Measurement Act 1961 and the RVS of position for GDA94 will be repealed.  Access to GDA2020 will be via the same methods applying to GDA94.

GDA2020 coordinates will be more closely aligned with the reference frameworks used by modern GNSS — such as GPS, GLONASS, Galileo and BeiDou — than GDA94 coordinates.  GDA2020 will supersede the existing GDA94 datum and older coordinate systems, such as Australian Geodetic Datum 1966 and 1984 (AGD66 and AGD84).

Leaders of government agencies responsible for spatial information in each jurisdiction have agreed to adopt the GDA2020 datum with transition to occur in the years following the release of the datum in January 2017.  Where appropriate this will include changing Commonwealth, State and Territory Acts and Regulations which determine the datum used for many dealings between Government and the community.

The definition of the GDA2020 datum includes fixed coordinates values gazetted for geodetic reference points within Australia.  GDA2020 position values, therefore, appear fixed to the tectonic plate and the coordinates appear to be unchanging with time.  Thus GDA2020 is a so–called “plate–fixed” datum and the relationship of positions with respect to each other on GDA2020 will not change significantly due to plate motion.

The Australian plate is moving in a north–north–easterly direction at a rate of about 7 cm per year, or about 6mm per month.  This is coupled with a small deformation within the plate and a slight rotation.  That means accurately measured latitude and longitude of points in Australia, relative to a realisation of an “earth-fixed” global reference framework such as WGS84 or ITRF also change slowly with time.

The relationship between WGS84⁄ITRF2014 and GDA2020 coordinates changes over time.  There are two reasons:

  1. WGS84 is recalculated on a world-wide basis twice a year for GPS reference stations.
  2. Tectonic movements of the Australian plate result in changed WGS84⁄ITRF2014 coordinates for Australian stations, but do not affect the GDA2020 coordinates.

The difference between GDA2020 and WGS84⁄ITRF20014 coordinates will be approximately 21 cm in January 2017, move to being approximately 0 cm in January 2020, then 21 cm in 2023, and then continue to diverge by approximately 7cm per year.

It won’t — it is a “plate fixed” datum, like GDA94 before it.  In the near future, precise point positioning systems which measure absolute positions to centimetre precision based upon the global “earth–fixed” ITRF reference frame will be commonly available.  This is why the ATRF is proposed — to provide an Australian reference framework that is able to represent the gradual change of absolute positions caused by tectonic plate motion and intraplate deformation. 

The Australian Terrestrial Reference Frame (ATRF) will be available from 2020.  This will be an “Earth–fixed” reference frame — as Australia moves with respect to the framework the coordinates of reference points will be observed to change over time. It will accommodate coordinates that incorporate the date of measurement within the coordinate reference system — that is, the date will be an integral component of the coordinate set. 

The GDA Modernisation Implementation Working Group will be developing plans for the implementation of the ATRF in 2017.

The general datum definition is the same between GDA94 and GDA2020; it refers to the same ellipsoid and both are based upon an ITRF realisation.  Geocentric Datum of Australia (GDA) has been retained in the name to indicate that it simply an improved version of the national datum, meaning it will have automatic recognition as the national datum amongst non–specialist users of spatial information. 

GDA94 is based on the geographic location of Australia on 1 January 1994.  By the year 2020 Australian continent will have moved approximately 1.8 metres to the north–north–east due to tectonic motion.  Thus the GDA2020 latitude and longitude coordinates in Australia will change approximately 1.8 metres from the GDA94 latitude and longitude coordinates in a north–north–easterly direction.  The exact change and orientation of the change will slightly vary from area to area.

Although GDA94 is nominally used as a 2D geodetic datum to denote the latitude and longitude of locations it is defined in three dimensions, as will be GDA2020.  The ellipsoidal heights between GDA94 and GDA2020 will differ by approximately 0.1 metres.  In all but the most exacting scientific pursuits, or for most practical purposes, the difference will be able to be treated as a constant offset.

If the accuracy of your horizontal spatial data is less than approximately 3 metres then the dataset can be considered GDA2020 compatible, since the 1.8 metre difference between GDA94 and GDA2020 is well within this currently routinely achievable GNSS measurement accuracy. 

GDA2020 compliant spatial data is information which has been measured with direct reference to the GDA2020 datum.

Map Grid of Australia 2020 (MGA2020) is a metric rectangular grid coordinate system.  It is a 2–dimensional Cartesian coordinate system based on the Universal Transverse Mercator (UTM) projection system and the GDA2020 datum.  The UTM projection system is a special case of the Transverse Mercator projection since the unit of measure (the metre), zone numbers, widths, central meridians, central scale factors and false Easting & Northing are all included in its definition.  

Similarly, the Map Grid of Australia 1994 (MGA94) was based upon the UTM projection system and the GDA94 datum.  Because both MGA94 and MGA2020 are UTM projections, the defining parameters for both grids are identical. 

The difference between MGA94 and MGA2020 Easting and Northing coordinates will also be approximately 1.8 metres and will simply reflect the projected differences between the GDA94 and GDA2020 latitudes and longitudes.

The UTM projection system is a special case of the Transverse Mercator projection of worldwide application between 84 degrees north and 80 degrees south latitude.  Since its definition includes the unit of measure (the metre), zone numbers, widths, central meridians, central scale factors and false Easting & Northing it allows users to unambiguously identify 2D locations in this coverage area and accordingly it is supplied in virtually all GNSS devices and spatial software as an option for displaying coordinate locations.  The location and shape of the Australian landmass (large longitude extent, primarily low to mid-latitudes) is also well represented by the UTM projection system.

It is not possible to change any of the defining parameters of MGA2020, such as the false Easting and Northing, since they are part of an international standard definition - if new parameters were adopted the projection would no longer be a UTM projection.  Accordingly, equipment and software manufacturers would have to specifically include the new, bespoke MGA2020 defining parameters. Not only would this take many years, and require a considerable ongoing communication effort with international software and hardware suppliers, ICSM considers that moving away from a UTM projection that has been applied in Australia for over 60 years would create more confusion and issues than the alternative.

Legal boundaries on land are generally defined by physical features or marks on the ground.  These boundaries will not change, though the coordinates of them may.

Offshore boundaries may have been defined by coordinates in terms of older datums.  These boundaries too will not change, although the coordinates will be different when converted to GDA2020.  Some boundaries have been defined by coordinates without reference to a coordinate system.  In these cases the physical location has always been uncertain and this predicament will continue until that doubt is resolved in each individual case.

The difference between GDA2020 and GDA94 ellipsoidal heights will be approximately 0.1m. 

Given that GNSS derived ellipsoidal heights are being increasingly used (particularly in Lidar surveys) this is important to consider when combining and comparing data.

The national vertical datum normally used for survey, positioning, construction and legal purposes within Australia is the Australian Height Datum (AHD). It is based on mean sea level measurements.  The change to GDA2020 will have no impact on AHD but it will impact on the models used to transfer between ellipsoidal heights and AHD.

The long term future of the AHD and possible transition to a new national gravity based vertical reference frame will be determined after the release of GDA2020. 

Ellipsoidal heights differ from AHD heights by between -30 and +70 metres across Australia. To convert ellipsoidal heights on the national geocentric datum to AHD heights (and vice-versa) an AUSGeoid model can be used.

Australia's current model for converting GDA94 ellipsoidal heights to AHD heights is AUSGeoid09. It is accurate to 0.03m across most of Australia.  There will be no new versions of AUSGeoid09 produced and AUSGeoid09 will not be compatible with GDA2020.

Given that GDA2020 and GDA94 ellipsoidal heights differ by approximately 0.1m a new model called AUSGeoid2020 will be developed to convert GDA2020 ellipsoidal heights to AHD heights. 

AUSGeoid2020 will be continually improved, and new versions released, as new data is included in the model.

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