Web Mapping and WGS84 / Web Mercator
WGS84 published on the 'Web Mercator' projection (EPSG:3857) have become the de-facto standard for publishing web maps and exchanging data for use in web mapping. The reasons for this are both historical and practical in relation to the need for a global frame of reference, the use cases for the mapping and the lower accuracy of both datasets and real time positioning that prevailed.
Whilst it is now internationally recognised that changes are required to web mapping to enable delivery of high accuracy maps and data for use in conjunction with real time positioning the 'WGS84 Ensemble' will remain the default 'datum' for web mapping delivery for the immediate future.
In June 2020 ICSM's GDA2020 Implementation Working Group published a detailed advisory on the consequences of the use of "WGS84" for web mapping and the work being undertaken to resolve the issues it causes.
What is the WGS84 Ensemble?
The complications for high-accuracy positioning and data in web mapping arising from using WGS84 result from two main aspects :
- Although WGS84 is a "dynamic" reference frame or datum it is applied within web mapping as if it is a "static" datum.
- WGS84 is not just a single "datum" - it is a collection of 6 signficantly different realisations that are all simply referred to, and understood in the international mapping community, as the singular "WGS84".
This inherent ambiguity in WGS84 locations has not been a concern until relatively recently
as it had no discernable impact on the vast majority of use cases which utilised positioning whose accuracy was only 3 metres or worse. However, the rapidly increasing availability of high accuracy positioning systems which is being used in conjunction with high accuracy web deliverered spatial data has resulted in the need to update web mapping practices and the international standards underpinning them to better accomodate this use.
A significant milestone in these updates was the 2019 inclusion of the concept of a "datum ensemble" in the international standard for spatial referencing by coordinates (ISO 19111:2019), allowing WGS84 to be described as a datum ensemble in the upgraded EPSG geodetic registry. This change aims to support the continued use of the now ubiqituious Coordinate Reference System EPSG:3857 (and the WGS84 ensemble datum EPSG:6326 upon which it is based) in web mapping, whilst a new approach supported by international standards that permits proper treatment of time-dependent location information in web mapping is developed.
For further information about dynamic datums, datum ensembles, high accuracy data and real time positioning watch this short video produced by the Geomatics Committee of the International Association of Oil and Gas Producers (IOGP) who are responsible for the EPSG registry.
Land Tasmania Web Mapping Solution
After considering the relevant technical and practical challanges associated with changing web mapping products in the current standards paradigm and the critical requirement to maintain direct inter-operability with all other web mapping services and web mapping users Land Tasmania has determined that LISTmap and LIST web services will continue to publish in WGS84 Web Mercator that is directly equivalent to GDA94 until such time as ICSM recommends an agreed alternative that achieves interoperability.
Demonstrating the misalignment of GDA94 and GDA2020 data when transformed into WGS84 for web mapping
Implications of this decision:
- Consumers of LISTmap and the web services will see no change in location / coordinates of features post 30 September 2020 - the coordinates are effectively GDA94 coordinates.
- Users who wish to mix and match GDA2020 datasets in a web-mapping platform will need to first transform from GDA2020 to GDA94 (i.e. 'pivot' through GDA94) before intergrating with Land Tasmania web services. Otherwise misalignment of data will result and as the misalignment is approximately 1.5 metres in Tasmania it will be visually apparent at higher zoom levels.
- Similar to above, destop GIS users consuming LIST web services who are working in a GDA2020 project will need to apply a transformation to the WGS84 Web Mercator based web services that pivots through GDA94.
- Some versions of software do not cater for pivoting around GDA94 in the transformation and in such instances the version will need to be upgraded to cater for this. Whilst it is the user's responsibility to consult their software support provider on this issue Land Tasmania can provide some limited information specific to some software packages. Contact listhelp for more information.
- Users of accurate real time positioning in GDA2020 or WGS84 @ current epoch on a mobile device may observe an apparent 1.5 metre misalignment between the observed location and the GDA94 based WGS84 Web Mercator datsets in Tasmania (This will depend upon the configuration settings in the device).