datum shift transformations have been fixed in qgis-1.8, which may explain this. This has been addressed in this list (and also qgis-dev) in the last months
Etienne On Thu, May 10, 2012 at 4:04 PM, Nick Hopton <[email protected]> wrote: > > 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 _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
