Progress supporting GDA2020:
The Intergovernmental Committee on Surveying and Mapping (ICSM) & Geoscience Australia (GA) recently released the definitions for the GDA2020 datum and associated projected coordinate systems covering Australia and its territories. Esri Australia has been working closely with ICSM & GA to ensure that ArcGIS software products support the new definitions. Version 10.6 of ArcGIS and version 2.1 of ArcGIS Pro both include support for the new datum and projected coordinate systems (you can learn more in the Esri Australia Technical Blog)
Avoiding alignment issues mixing datums:
Web GIS, by default, displays data in the projection WGS84 Web Mercator (auxiliary sphere). When mapping data is shared to ‘Web GIS’, data is automatically reprojected ‘on-the-fly’ to this web projection in order to align with the displayed basemap and fit with the global projection. Most users do not realise that their data is being reprojected and in many instances occurs without issue.
However, while organisations are migrating to GDA2020 from GDA94 and using mixed data containing both datums, these mixed data sources will create a challenge when displayed in WGS84. The issue occurs when data is reprojected to the WGS84 Web Mercator projection.
During the early 1990s, the governing bodies defining the transformation parameters between GDA94 and WGS84 set the transformation as ‘null’. At this period in time both GDA94 and WGS84 were considered coincident. That ‘Null’ parameter now creates a problem when projecting GDA2020 data to WGS84 Web Mercator as the differences between GDA94 datum (a static datum) and WGS84 (effectively, a dynamic datum) have increased over time. Essentially, the problem presents itself as a misalignment of GDA94 and GDA2020 data when viewed in a WGS84 based projection.
Today’s current governing bodies, ICSM, GA and the Global EPSG working group are working towards a permanent solution to resolve the reprojection dilemma. In the interim, Esri Australia recommends that if you are using data from both GDA94 and GDA2020 together, that both data sets be presented in either GDA94 or GDA2020 and not reprojected on-the-fly into WGS84. In ArcGIS, you do this by utilising the projection toolset. Your GIS staff should know how to perform this although they are welcome to call Esri Australia directly and assistance will be provided.
By transforming all the data to a common datum, the data sets will align with each other, and can then be shared without issue using a WGS84 Web Mercator projection
If you are interested in further technical details, I’ve included more in the section below…
Further Technical detail explaining the issue at hand:
The problem with the way WGS84 has been implemented is one which must be well understood by all who manage 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 flourished as a de-facto datum for web mapping because for “…practical applications of mapping, charting, geopositioning, and navigation” WGS84 works when sub 5 meter accuracy is not important.
This expectation around accuracy was fine when Google Maps launched in 2005. But expectations have changed, and the use of WGS84 in the web mapping environment needs a serious re-assessment. The problems stem from the way in which transformation to WGS84 from plate fixed datums has been implemented — as if WGS84 were a plate-fixed datum with a single time 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 Defence (DoD).
The transformation to WGS84 in the web mapping environment results in several limitations when using WGS84 in Australia that users should be aware of:
- Generally, software may suggest that it can perform the transformation but what it is really doing is a NULL transformation due to the establishment of null transformation parameters in 1994 when GDA94 and WGS84 were coincident. Hence, the commonly heard (incorrect) comment that WGS84 and GDA94 are interchangeable.
- 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.
- As a result of the NULL transformation when GDA94 data is projected to WGS84 Web Mercator (auxiliary sphere) and GDA2020 is projected to WGS 1984 Web Mercator (auxiliary sphere) there will be a 1 – 2 metre misalignment. This is described in the diagram below.
To sum up, when your organisation moves to GDA2020 and you have mixing of GDA2020 data with GDA94 data ensure that the display projection is one of these two datums. If you intend to display mixed datums over a WGS84 datum, then ensure the all data has either been transformed to GDA2020 or GDA94 first. This will ensure correct data alignment in the WGS84 web mapping geographic space.