Thursday 22 August 2019

Misaligned web-maps? High-accuracy data must become time-dependent

Misalignment of WGS84 web-maps when
sourced from GDA94 and GDA2020.
Current web-mapping implementations are compromising datum modernisation programs by introducing metre-level misalignments of data. It is the start of a suite of high-accuracy and time-dependent mapping challenges the world is about to face.

While web-mapping has worked ‘well enough’ to date, it cannot support the high-accuracy requirements expected by modern users and data. It is hampered by the ambiguity inherent in the practical application of WGS84 as used by the Web Mercator projection, and lack of care with the recording and use of time metadata.

Mass market positioning is now accurate enough to be affected by the earth’s measurable surface deformation and tectonic plate motion. Providing an accurate frame of reference for new users and applications has been a significant driver behind recent datum modernisation programs, for example in Australia and the United States. To succeed, these programs urgently require standards, software and practices to be updated and adopted.

In general, the time-dependence of spatial data is not yet properly managed and is often poorly understood and communicated. While standards such as ISO 19111:2019, ISO 19127:2019 and ISO 19162:2019 have been recently updated to address this issue, there remains significant work to refine:
  • Downstream spatial standards;
  • EPSG definitions for datums, projections, Coordinate Reference Systems (CRS) and transformations (changes are in train);
  • Software;
  • Published and stored datasets; and
  • Education and outreach.
These issues require international collaboration to be addressed effectively.

Key technical problem:

Web-mapping is generally published in the WGS84 Web Mercator Projection, which is based on the WGS84 datum ensemble (EPSG::6326). While this WGS84 ensemble represents any or all of the six formal realisations of the WGS84 dynamic (earth fixed datum), in practice WGS84 is generally used to mean either:
  1. The realisation (and epoch) most closely equivalent to the local national datum, for example when transforming accurate ‘ground-truthed’ maps and data into WGS84 for web-mapping. This usually relies on defined ‘null transformations’ to WGS84, as in Australia from GDA94 (EPSG::1150) and North America from NAD83 (EPSG::1188).
  2. The latest published WGS84 realisation, with the datum effectively static at the date of collection, as with single-point-positions determined directly via GPS, which also have an inherently low accuracy.
While each WGS84 realisation in the ensemble has been well defined, the generic use of the datum ensemble in web-mapping should be considered to have an uncertainty of several metres. 

The Australian datum modernisation initiative has exposed this uncertainty in WGS84 by introducing a new static national datum, ‘GDA2020’ at the epoch 2020.0. Coordinates in this datum are ~1.8 metre north-east of the previous national datum, GDA94, due to the 7cm per year tectonic motion of the Australian Plate. The new null transformation (EPSG::8450) from GDA2020 back to the WGS84 datum ensemble introduces systematic 1.8 metre web-mapping misalignments.

Datum modernisation programs planned for other regions will face similar problems, including when the U.S. introduces four new time-dependent datums in 2022.  To map these back to a common datum and epoch for web-mapping will require a much better definition of the web-mapping datum, and a better application of time-dependence in data and transformations.

Potential solutions:

It is recognised that a long-term solution is required to account for time-dependence in web-mapping. Options include:
  • Define a specific WGS84 realisation for web-mapping at a fixed and conventional epoch (point in time). 
  • Adopt and define a conventional epoch per region to reflect current practice in web-mapping. 
  • Avoid creating new null-transformations to the WGS84 datum ensemble.
  • Migrate away from the defacto WGS84 Web-Mercator projection currently used for web-mapping, to a new convention likely based on the International Terrestrial Reference Frame (ITRF), at a defined epoch.
  • Update web-mapping applications to natively cater for time-dependence. 
  • These options must be considered in concert with the required development of standards and practices to handle time-dependence across all spatial data applications.
In the short term, Australia still faces the immediate issue of 1.8 metre misalignments in web-mapping. The likely approach is to deprecate the newly introduced null transformation from GDA2020 (EPSG::8450) and replace it with a transformation which brings all local data back into alignment with the previous GDA94 datum, and with existing WGS84 web-mapping products, at epoch 1994.0. This would effectively acknowledge that WGS84 is being used in practice as a relatively accurate, static datum.

Support for this short-term approach would be required from software providers, who would have to introduce the custom GDA2020 => GDA94 => WGS84 concatenated transformation as default, and/or to adopt any new EPSG updates at the first practicable opportunity.

Call to action:

While there are a few quick-win solutions, most of the challenges will be hard. They will require deep insight, strong leadership, sustained effort, and international collaboration to address them properly. Australia is facing these issues now, and the rest of the world are about to face them.
We are cataloguing these challenges and possible solutions on behalf of Australia’s Intergovernmental Committee for Surveying and Mapping (ICSM). A draft will be published within a couple of weeks, by Friday 30 August 2019. It will then be presented at the next OGC Technical Committee meeting, September 2019 in Banff, Canada.
We want to test and refine these ideas with the breadth of the geospatial community and find collaborators. If you can help then please:
  • Comment on the draft document when it comes out.
  • Contribute to discussions currently happening on the OSGeo proj email list.
  • Reach out to us at the technical committee meeting if you will be there.
  • Take this message to stakeholders you think should be involved.
We look forward to hearing from you.

About the authors:

  • Joel Haasdyk is a Geodetic Surveyor, and GDA2020 Implementation Manager at NSW Spatial Services, Australia. He will be presenting these ideas at the OGC TC meeting in Banff on behalf of Australia’s Intergovernmental Committee for Surveying and Mapping.
  • Cameron Shorter is a Geospatial Business Analyst for the GDA2020 datum modernisation program at NSW Spatial Services, Australia; he is a long-standing OSGeo Open Source community contributor and commentator.

An early version of this document was published on the OGC Blog and more details can be found here.

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