Andy Harfoot wrote > > Have you tried the convertor here: > http://gps.ordnancesurvey.co.uk/convert.asp > to see whether the higher accuracy coordinates obtained from QGIS match > those obtained using OSTN02? >
For the record, the Survey gives the following (ETRS89) lat/lon coordinates for the points: Inkpen: 51.3524757 -1.4650872 Hodmore: 51.4984893 -1.0170283 Hindhead: 51.1154179 -0.7157938 I think ETRS89 has drifted about half a metre away from WGS84 at the present time, but even so the transformations done by Lisboa under Ubuntu look very good. > It looks as though Proj4 can use grid based transformations like OSTN02 > through the use of gsb files. Could it be that this is the case in the > higher accuracy situation? I find gsb files at the following location on > my Win7 QGIS 1.7.4 install: > C:\OSGeo4W\share\proj > At present by default QGIS doesn't use a grid-shift file for EPSG:27700. The datum-shift is carried out by the +towgs84 part of the Proj parameter string. > I can't see any files in this directory suggesting they are the OSTN02 > grid though, and I can reproduce your 'Vista' results as follows, > suggesting that my QGIS is using the lower accuracy seven parameter > transform. > It's not using a grid-shift file. But it's interesting that you get the same poor transformations that I get with 1.7.4 under Visa. Lisboa RC1 under Ubuntu produces much better transformations (for me) but I don't know why this is the case. Nick. -- View this message in context: http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-tp4965339p4966416.html Sent from the Quantum GIS - User mailing list archive at Nabble.com. _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
