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

[Even Rouault]

Selon Andrew Chapman <andrew.chapman at donkagen.co.uk>:

> 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?

There's a GDAL ticket about that : http://trac.osgeo.org/gdal/ticket/4597

Frank, I've attached in it a libgeotiff patch that should solve this by defining
an override.