GDA 2020 - "Know your data, Know your datum"

The Australian and New Zealand Spatial Information Council (ANZLIC) set 30 June 2020 as the date by which its member agencies in all Australian states and territories will be ready to deliver and receive foundation spatial data on the GDA2020 datum.

The South Australian Government GDA2020 Implementation Working Group (SAGGIWG) are pleased to announce that its member agencies have met that commitment and are able to deliver and receive spatial data in GDA2020.  This is a key milestone in the delivery of the state government’s implementation of GDA2020 as the operational datum for South Australia’s spatial data.

This doesn’t mean data is no longer available in GDA94, it means each agency is now able to provide data in GDA94 or GDA2020 at the request of their downstream user.  Similarly, agencies are still able to receive data in GDA94 and each agency has its own policies and guidelines with respect to the provision or consumption of its spatial data.  Please check with the specific agency you deal with to find out more.

Over the next 12 months, SAGGIWG will continue to work towards making GDA2020 the native datum for all South Australian spatial data.

Over the next 12 months, SAGGIWG will continue to work towards making GDA2020 the native datum for all South Australian spatial data.

Several state government agencies have released their open spatial datasets in both GDA94 and GDA2020.  Data is available for download as a zip file containing _GDA94 and _GDA2020 datasets allowing users to choose the datum of the dataset they wish to use.

“Know your data, know your datum”

With some spatial data now available in two datums, it is imperative to “know your data, your datum” and note the metadata of imported datasets to ensure alignment with existing data.

ICSM Metadata Working Group have produced an informative paper titled “Preparing metadata for the Australian Geospatial Reference System” to not only provide guidance for preparing metadata for GDA2020 but also for the Australian Terrestrial Reference Frame (ATRF) and the Australian Vertical Working Surface (AVWS).

With few exceptions, SA Government spatial datasets have been re-projected from GDA94 to GDA2020 using the Conformal + Distortion NTv2 transformation grid developed by Geoscience Australia.  Please refer to the ICSM website to understand the differences between transformations.

If it is unclear which transformation parameters have been used to transform the dataset please contact the respective agency’s “contact point”.

Web Mapping

There are known challenges with mixing GDA94 and GDA2020 data through the web mapping medium. The issue originates from early days of web mapping when WGS 84 and its common ‘Web Mercator’ projection were adopted globally as the default datum for web mapping

In 1994, GDA94 and WGS 84 were considered equivalent, so null transformation parameters between GDA94 and WGS 84 were introduced.

Since WGS 84 is a ‘dynamic’ (or ‘time-dependent’) datum in which coordinates of features slowly change as a result of ongoing tectonic motion, this equivalence with the ‘static’ GDA94 slowly degraded until it remained accurate only at the metre level. However, since WGS 84 was generally used for low accuracy positioning or web-mapping applications (metre level or greater) no processes were compromised.

When GDA2020 was defined almost 25 years later, a similar process was employed to create another null transformation between GDA2020 and WGS 84, since WGS 84 was now equivalent to GDA2020.  As a consequence, the EPSG registry describes GDA94 ≈ WGS 84 ≈ GDA2020 for low accuracy applications. This equivalence doesn’t hold for higher accuracy applications where there will be a misalignment of approximately 1.6m.

In the short-term, to overcome the misalignment issue, state government web mapping environments will supply data explicitly as WGS 84 equivalent to GDA94 as this approach is intended to support the significant amount of data currently available in that format.

Need more information?

For more information on GDA2020 including fact sheets and transformation tools visit the ICSM website.

Please direct any queries regarding the South Australian Government’s implementation of GDA2020 to bradley.slape@sa.gov.au.