Whilst performing some vector work, a staff member at Environment Systems noticed that features had become misplaced after upgrading to QGIS Long-term Release v3.10. Senior GIS Analyst, Matt Yandell-Thomas confirmed the problem and opened a bug with QGIS (https://github.com/qgis/QGIS/issues/34993). The developers quickly concluded that QGIS itself was probably not the actual cause, but rather some underlying library, and closed the issue. Anthony Scarth (Earth Observation Developer) set about building various combinations of QGIS and library versions (e.g. PROJ, GDAL, etc.), to see which piece of software was likely to be causing the issue, to target further investigation. It was concluded that the upgrade to GDAL v3 (the library responsible for raster and vector handling) was in some way responsible.
A discussion with the developers led to the conclusion that EPSG:27700 used to have a different definition in previous versions of GDAL. Any OSGB datasets that had been created before this would now have an “unrecognised” projection. A developer initially coded a script to apply a fix to each “old” dataset so that it could be used correctly in the latest QGIS, but this would have been incredibly time-consuming to run on all datasets, and not viable for users who weren’t aware of this issue, or had no experience of running scripts. We pressed for a more long-term solution and, thanks to help from the wider community (particularly Peet Whittaker in the above issue tracker), and a later report that this was also affecting a range of other projections, the developers integrated a fix into GDAL v3.1. Now all pre-existing datasets maintain automatic compatibility going forwards, and are positioned correctly. We’d like to thank core GDAL developer Evan Rouault for providing the code to fix this problem.
Environment Systems is pleased to be able to support QGIS, and many other open-source communities. As a power user of QGIS, our years of experience help us give back and do what we can to help make the software even better.
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