WGS84 conversion to EPSG:27700 in error by greater than 100 metres

[nhopton]

OSGB (EPSG: 27700) CRS Problem. Version 1.0.0 (II)

When reprojecting a WGS84 shape file to EPSG:27700 (Ordnance Survey of Great Britain) on the fly in QGIS I noticed errors of around 125 metres in the conversion. This is quite serious for users in the UK. Error in conversion of this magnitude are usually the result of the conversion not taking into account the fact the WGS84 datum and the OSGB36 datum are not one and the same thing (but the conversion does use the +datum=osgb36 parameter, I don't understand).

For the time being I've defined my own OS CRS using the following parameters:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.999601 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 +units=m +no_defs no_defs <>

Which converts from WGS84 to EPSG:27700 good to a metre or so. I've had this problem with other software that uses Proj.4.

[pcav]

See also #377 and #1079, all probably related

[pcav]

Isn't this really a PROJ4/EPSG bug? Please report it there.

[pcav]

If the same error happens with all PROJ.4 software, it should be a PROJ.4 issue, not a QGIS one. Please reopen the ticket here if necessary