OS Terrain 5 is supplied as a whole set of Great Britain divided into 5km-by-5km tiles. These tiles are identified by quoting the National Grid reference of the south-west corner of the area they cover.
Ordnance Survey divides Great Britain into squares 100km-by-100km. Each of these squares has a unique two-letter reference, for example, TG in the diagram below.
Map of Great Britain, divided up into 100km-by-100km squares with National Grid reference identifiers.
To describe a 10km-by-10km tile, first add a two-digit reference to the 100km-by- 100km square reference, with the easting first followed by the northing: for example, TG23.
Then identify which quadrant of that grid square is required and add SW, SE, NW, or NE to the reference, for example, TG23SW.
