On 11 October 2017, Dr R. Bruce Warrington, Chief Metrologist with the National Measurement Institute published "Determination 2017" with the effect that GDA2020 is now the Recognized-value standard of Measurement of Position in Australia (i.e. it is our new legal datum.) As far as Victoria is concerned, we are yet to adopt this new definition into Victorian legislation, policy and Surveyor-General Practice Directives. Strictly speaking, until this occurs, the official datum for Victoria will still be GDA94; Victorian government legal and policy experts are working to make the necessary changes as quickly as possible.
GDA2020 represents a horizontal shift in Victoria of approximately 1.6 m north-east compared to GDA94.
GDA94 is not disappearing but its usage will begin to decline as the business value of high-accuracy spatial data is realised by organisations.
Using Transformation Grids to Handle Local Distortion
The Earth's landscape is constantly changing from natural (e.g seismic events) and man-made causes (e.g. over-extraction of groundwater) that can have effects at different spatial scales. If - and how - jurisdictions have accounted for these changes in their realisation of GDA94 will determine whether they can transform to GDA2020 with or without considering a distortion component for their data.
Some states will be distortion-free, while others will need to account for conformal changes as well as distortion. In Victoria, border areas and certain towns display up to 0.2 meters of distortion when users apply the 7-parameters alone when transforming data from GDA94 to GDA2020; when using the parameters and a transformation grid, this distortion is removed.
The ICSM has published two transformation grids for all of Australia so that users can transform from GDA94 to GDA2020 regardless of where they are and without needing to consider possible implications from local or regional distortions. These transformation grids support consistent, lossless transformations to and from GDA2020.
That said, if your data is at an accuracy-level that makes distortion irrelevant (anything worse than 1 meter), then you won't need to use a transformation grid at all - the 7 parameters will be sufficient.
Geocentric Datum of Australia 2020
GDA2020 Geodetic Datum
Where to Begin with GDA2020
If you work "a bit" with spatial data:
The first of the GDA2020 Datum animation videos, Modernising Australia’s Datum, provides a clear, short introduction to the importance of datums and the reason behind the change to GDA2020. The video is intended for clients, customers, student-groups and others less familiar with spatial matters, but who need or want to grasp the importance of the change.
Geoscience Australia has developed a web service powered by FME that transforms selected spatial format files from AGD66/AGD84 to GDA94 and from GDA94 to GDA2020 using the published ICSM NTv2 grids. Users can simply “drag and drop” files onto the page and receive an email with a link to download the output file. This service enables users to test the outputs of limited formats to ensure that your GDA2020-enabled software is outputting the correct coordinates.
There is also an Online forum (http://gda2020.invisionzone.com/) where you can discuss software, hardware, or industry-specific issues with other users across Australia, as well as get questions answered about the Technical Manual or other official resources provided. The online forum will help to establish a peer-to-peer community where you can get authoritative answers and share your experiences.
If you are a surveyor or other professional requiring a deep technical understanding:
descriptions, transformation parameters and examples to assist with datum transformations between GDA2020, realisations of the International Terrestrial Reference Frame (ITRF) and historic Australian datums;
descriptions and examples to assist with coordinate conversions between Earth - centred Cartesian, geographic and map projected coordinates;
descriptions and examples of coordinate computations; and define the Australian Height Datum (AHD) and AUSGeoid2020, and describe how to convert between ellipsoidal heights and AHD heights
How do I know if my software is really transforming to GDA2020?
It is critical that all systems and applications produce the right numbers when transforming data between datums and projections. With the introduction of a new datum such as GDA2020, early adopters must be particularly judicious in ensuring that their software is tested and producing the right outputs.
A dataset of tested outputs has been produced by the Office of Surveyor-General Victoria at the Department of Environment, Land, Water and Planning specifically for this purpose in Victoria and is available (from January 15, 2018) to download through Spatial Datamart (search for GDA94TOGDA2020_SAMPLE_DATA.) The dataset includes a sample of 48 survey marks across Victoria in Vicgrid, GDA94 and GDA2020 coordinates, including marks from areas that are impacted by significant distortion. Coordinates are displayed in decimal degrees (latitude and longitude) and metres (ellipsoidal height, Easting and Northing)
The dataset includes GDA94, VicGrid, and GDA2020 coordinates derived in different ways:
GDA94 coordinates from the Victorian Survey Control Network Adjustment
GDA2020 coordinates from the National Adjustment
GDA2020 coordinates derived by applying the conformal grid to the GDA94 coordinates
GDA2020 coordinates derived by applying the distortion + conformal grid to the GDA94 coordinates
GDA2020 coordinates derived by applying the 7 parameter transformation to the GDA94 coordinates
Additionally, Geoscience Australia has developed a web service available at http://positioning.fsdf.org.au/ that will transform selected spatial format files from AGD66/AGD84 to GDA94 and from GDA94 to GDA2020 using the published ICSM NTv2 grids. Users will be able to simply “drag and drop” files onto the page and receive an email with a link to download the output file.
Q: Do I really need to know about GDA2020 if all of my data is in WGS84?
The problem with the way WGS84 has been implemented is one which must be well understood by all who manage spatial information; it is both a cautionary tale as well as a significant barrier that may prevent users from realising the value of enabling high-accuracy spatial information.
For nearly 20 years WGS84 has been treated as the "universal standard" for spatial referencing across the globe and, as such, its ubiquity makes it one of the strongest brands in the spatial domain. In particular, it has flourished as a de-facto datum for web mapping because for “...practical applications of mapping, charting, geopositioning, and navigation” WGS84 works when accuracy of greater than 5 meters is not important. This expectation around accuracy was fine when Google Maps launched in 2005.
But expectations have changed, and WGS84 needs a serious re-assessment.
The problems stem from the way in which WGS84 has been implemented -- as if it were a plate-fixed datum with a single realisation (which it isn't).
The World Geodetic System (1984) is the reference frame used by the Global Positioning System (GPS) and developed by the USA Department of Defense (DoD). It is now maintained by the U.S. National Geospatial Intelligence Agency (NGA). It is a semi-dynamic reference frame. Since it was first introduced it has been revised five times, with the most recent version (G1762) implemented on 16 October 2013 and aligned to the ITRF2008. This means WGS84 coordinates derived from GPS will appear to move over time unless they are updated. Updating them requires knowledge of what epoch (e.g. the year) the coordinates were collected in. However, quite often, GPS coordinates for WGS84 are collected without an explicit epoch being recorded, making it impossible to know which ITRF realisation should be used. For example, if your data is collected between 2013 and 01/01/2018, the epoch would be ITRF2008@2017.5.
This implementation results in five main limitations with using WGS84 in Australia that users should be aware of:
There are no official tools for transforming to or from current (GDA2020) or legacy (GDA94) Australian geodetic datums to WGS84. (The software may suggest that it can but what it is really doing is a null transformation.)There are pathways from GDA to WGS84(XX) via ITRFXX for the more inventive, however this is not straight-forward.There are multiple entries for transformations from the various WGS84’s to WGS84’s and WGS84’s to ITRFXX’s for those who are keen.
Unlike the Geocentric Datum of Australia 1994 (GDA94), WGS84 does not have a recognised value standard for measurement of position under the National Measurement Act 1960 in Australia. This is not to say that WGS84 is illegal, only that it will be harder in a court of law to prove where a WGS84 coordinate is physically on or above the ground than a GDA2020 (or GDA94, AGD66/84) or ITRF position.Having a recognised-value standard for GDA2020 provides this ready-made proof.
The accuracy of WGS84 (G1762) as realised using the consumer broadcast technologies is now 2-5 meters. Broadcast technologies update the coordinates of their tracking stations each year (currently at 2017.5). However, receiver technologies must also be updated and this will depend on factors including user and receiver capabilities, as well as maintenance licenses that may be in place. So a GPS position is subject to both the receiver error and the epoch of the observations. For civilians up until now, receiver error has typically swamped epoch difference.
Australian users are not generally able to achieve better accuracies as they cannot access WGS84 via a differential measurement directly to the DoD network of stations. Given points 3 and 4, WGS84 data can have a positional uncertainty of no better than 5 meters.
Since Australia is moving up to 7cm per year, WGS84 coordinates collected 20 years ago will have experienced 1.4 meters of apparent horizontal shift.
Since there is not a universally accepted methodology (yet) for handling the transformation to/from WSGS84, in Victoria we have been suggesting that users adopt the following steps:
Identify the date/time (or epoch) when the WGS84 data has been captured. If the data was captured by GPS point positioning, then an epoch of @capture_year.5 needs to be used in Step 3, below. For example, you say “WGS84 (G1762) aligned with ITRF 2008 @ 2005.0 (no change) - Adopted in Oct 2013” but if the data capture is in 2015 then it is ITRF2008@2015.5 not 2005.0.
Identify the version of ITRF to which WGS84 was aligned at the epoch of data capture.
Set this version of ITRF and epoch as the underlying datum of the dataset.
Perform the transformation to GDA94 or GDA2020 using the published parameters.
The key take-away to all of this is that, if you are looking for a spatial reference system that will enable the highest positional accuracy for coordinates in Australia, GDA2020 is the best there is.