With regard to the discussion on the best transformation, OSM recommends that all transformations from OSGB36 to WGS84 should be carried out using a 3-parameter transformation which is accurate to 21 metres. (https://wiki.openstreetmap.org/wiki/Converting_to_WGS84#Great_Britain <https://wiki.openstreetmap.org/wiki/Converting_to_WGS84#Great_Britain>). Is this still the recommended transformation?
There are 4 different transformations commonly used to transform from OSGB36 to WGS84//ETRS 89 The first method uses a 3-parameter transformation and is accurate up to 21m. (See https://epsg.org/transformation_1195/OSGB-1936-to-WGS-84-1.html <https://epsg.org/transformation_1195/OSGB-1936-to-WGS-84-1.html>) This is the transformation recommended by OSM (https://wiki.openstreetmap.org/wiki/Converting_to_WGS84#Great_Britain <https://wiki.openstreetmap.org/wiki/Converting_to_WGS84#Great_Britain>) The second uses a 7-parameter transformation (the Helmert Transformation) and is accurate up to 2m. (See https://epsg.org/transformation_1314/OSGB-1936-to-WGS-84-6.html <https://epsg.org/transformation_1314/OSGB-1936-to-WGS-84-6.html>) The third is OSTN02 which uses a NTv2 grid file and was accurate to centimetres but has now been superseded by OSTN15. The fourth is OSTN15 which also uses a NTv2 grid file (OSTN15_NTv2_OSGBtoETRS.gsb) and is also accurate to centimetres.(See https://epsg.org/transformation_7953/ETRS89-to-OSGB-1936-British-National-Grid-3.html <https://epsg.org/transformation_7953/ETRS89-to-OSGB-1936-British-National-Grid-3.html>) QGIS and other GIS software will generally use the OSTN15 (or OSTN02) transformation by default but only if they can find the NTv2 file. They will otherwise revert to the second 7-parameter transformation. Using the correct transformation has implications for everyone wishing to import open data sets such as UPRN and NAPTAN data. These data sets have their location data sourced in BNG (and are probably positioned with reference to OS Mastermap). When these data sets are openly published a convenient Lat/Long is also provided that has been transformed/reprojected from the original BNG. It should be noted that there is no consistency in the Lat/Longs that are provided by data providers, as they are free to use whichever transformation they choose. It should also be noted that OSM is mapped in WGS84 because OSM is a global map and WGS84 is a global CRS. The Ordnance Survey transformation on the other hand converts British National Grid (BNG) to ETRS89. However, ETRS89 is a continental CRS which moves 2.5 cm every year with respect to WGS 84. That is a different can of worms best left unopened. Do OSM contributors use the 3-parameter transformation recommended on the OSM wiki page? If they don’t, should the wiki page be updated to reflect current practice. Gareth
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