A chap on QGIS at Stack Exchange is having problems with accuracy when transforming from Ordnance Survey grid coordinates to WGS84 lat/lon. He's finding discrepancies in the order of seven metres.
Using QGIS 1.7.4, OSGEO4W under Vista I was able to reproduce the problem, but using QGIS Lisboa RC1 under Ubuntu 11.10 produced transformations good to better than a metre. My first reaction was that there must problems with the QGIS 1.7.4 Proj4 parameter string for EPSG:27700, but on checking I found that 1.7.4 and Lisboa use identical strings for this CRS. I'm now at a bit of a loss to understand how following an identical procedure on both systems can produce differing results. If anyone would care to check this, below is the csv file I used for my tests. Station,x,y Inkpen,437346.0,161624.8 Hodmore,468296.3,178184.9 Hindhead,489984.6,135909.7 Under Ubuntu these coordinates transformed to: Inkpen 437346 161624.8 -1.465102 51.352478 Hodmore 468296.3 178184.9 -1.017527 51.498487 Hindhead 489984.6 135909.7 -0.715814 51.115423 Under Vista they transformed to: Inkpen 437346 161624.8 -1.46511 51.352424 Hodmore 468296.3 178184.9 -1.017525 51.498435 Hindhead 489984.6 135909.7 -0.715808 51.115362 In my part of the world the differences amount to about six metres on the ground. The Inkpen test point is in fact an old triangulation pillar and the point as transformed by Ubuntu sits exactly on this when viewed in Goorgle Earth. Any ideas? Nick. -- View this message in context: http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-tp4965339.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
