Understanding GDA2020 and it’s relationship with Web GIS

NOTE: this was first published Dec 2018 and has been amended in June 2021 to reflect additional software and transformation updates. See note at end of blog.

Background

Australia sits on one of the Earth’s fastest moving tectonic plates which has been moving 70 millimetres per year. By 2020, Australia will have moved 1.8 metres north east of it’s location in 1994. To effectively map the earth, representations, known as datums are used to model and identify locations. Australia’s national datum; Geocentric Datum of Australia 1994 (GDA94) will soon be replaced by a new datum Geodetic Datum of Australia (GDA2020). GDA2020 will align with current positioning technology.

Geoscience Australia and the Intergovernmental Committee on Surveying and Mapping (ICSM) have released a new Geodetic datum GDA2020. The datum provides higher positional accuracy and will be able to represent locations dynamically rather than the stationary GDA94. .

In the coming two to three years most organisations will be transforming their spatial data from the GDA94 datum to the new GDA2020 datum. Esri’s, ArcGIS Software has been updated to accommodate these new datums across Australia and will support all of our customers rigorous requirements for locational and positional accuracy.

Effects of Web GIS

With the evolution of ArcGIS Online and the ArcGIS Enterprise platform, we have seen an explosion of content which organisations can consume to provide context for their authoritative datasets. Esri for example provides nine individual global basemaps ranging from imagery to street based data. This is ideal for organisations who require base map information. However, providing seamless global datasets has meant organisations such as Esri have had to settle on a datum that best fits the entire world. This is the World Geodetic System 1984 (WGS84). All Esri’s basemaps, scene layers and global display engines are now based on this datum.

For organisations; displaying authoritative data against Esri’s basemaps and in the 3D scene environments is a simple transformation. From GDA94 to WGS84 . Moving to GDA2020 this simple reprojection requires further consideration.

The move to GDA2020

GDA94 and WGS84 are  generally interchangeable with little effect on positional accuracy. Unfortunately, with the modernisation of the Australian datum to GDA2020 there is not a definitive relationship between GDA2020 and WGS84. This will result in any transformation between GDA2020 and WGS84 having an error of 3 metres or greater.

The following diagram gives a representation of the relevant issue:

So, what can be done

Data in the web map environment can be displayed against Esri basemaps and web scenes and other WGS84-based data. In most instances this will not be an issue as the generic basemap data is derived from many sources with varying positional accuracies.

From a user data perspective Esri Australia recommends:

1. 1. 1. Transform all data to GDA2020 and republish to Web GIS in GDA2020 or a GDA2020-based projected coordinate system.
      2. When publishing to WGS84, utilise the projection path GDA2020⇒GDA94⇒ Data will be published with GDA94 accuracy and coordinate transformations between WGS84 and GDA94 can be Performed.
      3. Create your own basemaps (Imagery, Street and Topographic) in GDA2020
      4. DO NOT mix data from GDA94 and GDA2020 in Web GIS as there will be a 1.8 metre alignment error regardless
      5. If coordinate conversion is required in Web GIS from WGS84 to GDA2020 then a transformation path of WGS84⇒GDA94⇒GDA2020 should be utilised

NOTE: in January 2021 ICSM released new transformations that support GDA2020 to WGS84 and Esri released ArcGIS Pro 2.8 in May 2021 which supports these. Please refer the following article for these updates  New GDA2020 transformations for WebGIS