[Qgis-user] EPSG:27700 - OSGB 1936 / British National Grid shapefiles are incorrectly interpreted by QGIS 1.8.0 and later

[Andrew Chapman]

I've been asked to look into a problem where our exported shapefiles are
being displaced approximately 5.7m SSW from their true position.

Everything works as expected up to and including 1.7.4, but 1.8.0 and Master
demonstrate the problem. KML files are ok as are shapefiles using other CRSs
(e.g. EPSG:3857 - WGS 84 / Pseudo Mercator). If, using 1.8.0, I save a
EPSG:27700 layer as a shapefile with a different CRS or as a KML file, the
displacement is exported and loads as such in earlier versions of QGIS or
external applications (Google Earth, ArcGIS).

In 1.7.4 the layer metadata shows
  +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+ellps=airy 
          +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m
+no_defs

while in 1.8.0 it shows
  +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+ellps=airy 
         +towgs84=375,-111,431,0,0,0,0 +units=m +no_defs

The '+towgs84' parameters have changed.

For anyone primarily using OSGB shapefiles (e.g. the approximately 10,000 UK
town and parish councils who could use free Ordnance survey maps and QGIS)
this leads to a major creditability problem if we wish to share data with
other organisations.

Has anyone else seen this problem?

Andrew Chapman