On Wed, 19 Aug 2020 at 15:36, SK53 <sk53....@gmail.com> wrote: > This isn't necessarily true. If you open any OS Open Data product in QGIS one > is now confronted by a bewildering array of ways of converting from the OSGB > national grid co-ordinates to WGS84. > > The optimum one currently uses the 2015 file of detailed offset corrections > to the basic projection transformation. There was an earlier set of similar > data released in 2002. If one doesn't download this correction data then it > falls back on the basic transform using OSGB36 which can be anywhere between > 1 and 5 m off-true. In addition there has always been the slightly obscure > behaviour of OSGB projections specified in proj4 or WKT formats with respect > to the Helmert Transformation parameters (in early days of Open Data Chris > Hill & I found these were essential). At least part of the problem is that > EPSG:27700 appears to relate to several very slightly diverging projections, > whereas, for instance, Irish Grid changes are handled by EPSG:29001 through > EPSG:29003, and Swiss Grid CH1903 is EPSG:4149, CH1903+ is EPSG:4150 and the > newest CH1903+/LV95 is EPSG:2096. > > I don't know what transformation JOSM uses when reading EPSG:27700 so unless > one is very cautious it is not possible to be certain that one is anywhere > near the RMS 25 cm accuracy of OS data (especially as products, including > Boundary Line, may be partially generalised.
Perhaps this is what's causing the following problem for me then. I GPS-surveyed a lot of the roads on my estate a few years ago when aerial imagery wasn't so good. I've got GPS traces in OSM that consistently follow the pavements on each side of the road and will line up nicely with the aerial imagery if you put in the right off-set. However, the required off-set for these traces is around 3m out from the offset you need to make the OS OpenMap Local Functional sites (as suggested above) line up, when I load the shapefile directly in JOSM. This ~3m is very noticable when you have mapped buildings and pavements sticking out into roads. Perhaps it would be useful if someone could to do a correct transformation of (e.g.) the OS OpenMap Local Functional Sites data to a format and CRS that can be unambiguously read by JOSM, in order to help our imagery alignment. Robert. -- Robert Whittaker _______________________________________________ Talk-GB mailing list Talk-GB@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-gb
