I can confirm that the Land Registry wms parcels appear to have been converted with the Helmert 7-element transformation (no look-up table). This gives a misalignment of up to 5 metres. It's ironic that the Land Registry don't seem to know where their parcels are to better than 5m.
Now we know what EPSG:27700 does. It does the above transformation. I agree with Rob that the misalignment of 5m is obvious if you look at Hugh Town (Scilly). Both if you compare with the OSM data and if you compare with the tracklogs that have been uploaded to OSM. So this transformation won't do. I think we need to go for the look-up table. I've done some testing with JOSM. The look-up table transformation is not in JOSM's list of (thousands) of projections. But this custom projection does it: +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +nadgrids=OSTN02_NTv2.gsb +bounds=-9,49,2,61 +no_defs I expect something very similar would work in Mapnik. When you set up this custom projection, JOSM downloads the grid file from the JOSM server, and puts it in the JOSM cache folder under a modified name. There is then a wait of several seconds while JOSM configures the custom projection. You can also get JOSM to do the latest, OSTN15 transformation. The only change needed, is to the grid file. This needs some simple hacking because it's not supported. You don't change the custom projection, but you alter the file in the cache folder. So, find the file, copy its name, and then delete it. Download the OSTN15 grid file from the OS website. As Gareth says, you need OSTN15_NTv2_OSGBtoETRS.gsb, (and not the other way round). Put the file in the cache folder and rename it to the name you just copied. You then need to quit and relaunch JOSM for this change to 'take'. The difference between OSTN02 and OSTN15 is a shift, mostly in longitude, and in a similar direction throughout GB, of 1-2cm. With the look-up table transformation, there will still be a misalignment of 0.75m relative to WGS84, but this is a lot better than 5m. If there is consensus, then the wiki needs to be updated to recommend the OSTN15 transformation. _______________________________________________ Talk-GB mailing list Talk-GB@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-gb
