British National Grid Shapefile - convert to WGS84 Lat/Lon

mapping coordinate-systems wgs84
Elliveny picture Elliveny · Oct 9, 2009 · Viewed 9.3k times · Source

I have a set of ESRI shapefiles which, I'm told, have been written using the British National Grid coordinate system. I need to convert these files to WGS84 lat/lons, for onward conversion to KML files. I'm having trouble doing this as follows:

If I open each of the original files in MapInfo Professional telling it that my file has a projection which is British National Grid then I can't see any geographic objects in the file; the map window is completely empty.

If I use MapInfo Professional's Universal Translator to convert the files to a WGS84 MapInfo TAB file then, just as before, the converted file won't display any geographic objects, the MapInfo window is empty.

Can I verify the coordinate system of these files? Am I missing anything here? Should I be able to convert the shapefiles in the way I'm expecting to be able to and view them using MapInfo Professional? Will another tool do a better job for me?

Thanks.

More Info:

My shapefile has coordinates which don't seem to translate to lat/lon properly and I'm now wondering if the coordinates aren't actually British National Grid. I'm seeing coordinates such as 383702523, 399081141 which appears to be approximately lat/lon 53.488182, -2.247153. Have you any idea what projection system my input file is in?

Answer

Martin Beckett picture Martin Beckett · Oct 9, 2009

OS grid doesn't use WGS84 - it uses Airy 1936 (OSGB36) spheroid
So you need to go from OSgrid -> lat/lon then OSGB36->WGS84.

See http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/convertingcoordinatesEN.pdf

To do OS grid to lat-lon see http://www.movable-type.co.uk/scripts/latlong-gridref.html

Then to go from OSGB36 -> WGS84 see http://www.movable-type.co.uk/scripts/latlong-convert-coords.html

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