Hi All, Posting this here as I hope someone can remember from when they were looking at the OS OpenData release:
I'm trying to work out if I am using the correct Coordinate Reference System for a shapefile downloaded from Natural England. The shapefile includes a .prj as follows: PROJCS["British_National_Grid",GEOGCS["GCS_OSGB_1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",400000],PARAMETER["False_Northing",-100000],PARAMETER["Central_Meridian",-2],PARAMETER["Scale_Factor",0.999601272],PARAMETER["Latitude_Of_Origin",49],UNIT["Meter",1]] This seems to match the info provided by the OS at [1] however the new.prj file at [2] and the details at [3] differ. Firstly [2] has a different airy ellipsoid and both also include the towgs84 parameters (Helmert transformation). For QGIS what projection do I need to select? I have been using EPSG:27700 [4] which matches my .prj file, but now I am confused about the towgs84 parameters. Do I need to take these into consideration or is the Natural England data different to the OS OpenData data? Thanks, RobJN [1] http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf [2] http://shapes.raggedred.net/ [3] http://wiki.openstreetmap.org/wiki/Converting_to_WGS84 [4] http://www.spatialreference.org/ref/epsg/27700/
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