Why might Ordnance Survey codes all be out by 500m?

Question

I've been given a set of UK Ordnance Survey geocodes, but I think they're all off by about 500 metres.

Here is an example - please could someone confirm to me that the codes are wrong, not my calculations?

The OS geocode that I've been given is SO7567, for a place called Abberley, in Worcestershire.

My script converts this to 52.30136731, -2.36798287 here: http://goo.gl/maps/9a3Yu However, Abberley is here, about 500 metres north: http://goo.gl/maps/OAD9L

This conversion site suggests my calculations are correct, and it is indeed the original coders (who looked at Abberley, and decided that its Ordnance Survey code was SO7567) who were out of whack.

Could anyone suggest why they might all be out by exactly this amount - could it be a projection issue? I'm not an geographer, and the person who gave them to me swears they should be right!

Thank you for nay help.

Answer 1

Your example grid reference only has 4 digits and thus specifies a location which is 1km square, so plenty of room for a 500m error there.

See Ordnance Survey grid digits for examples of numerical resolution. More digits will give you more accuracy.