Ordnance Survey divides Great Britain into 100km x 100km squares. Each of these squares has a unique two-letter reference, for example, TG in the diagram below.
OS VectorMap Local tiles are identified by quoting the National Grid reference of the southwest corner of the 100km² area they cover, for example, TG.
To describe an OS VectorMap Local raster format tile, which covers 5km x 5km, first add a two-digit reference to the 100km x 100km square reference, with the easting first followed by the northing, for example, TG23. This represents a 10km x 10km area which can be suffixed with a NE, NW, SE or SW to describe the 5km x 5km tile.
A general introductory guide to the British National Grid (BNG) is available on the OS website.
