# The National Grid

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.

<figure><img src="/files/e0WdctTbdvATkFksXcMr" alt="Map of Great Britain, divided up into 100km-by-100km squares with National Grid reference identifiers." width="563"><figcaption><p>Map of Great Britain, divided up into 100km-by-100km squares with National Grid reference identifiers.</p></figcaption></figure>

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.

For additional information on how to use the National Grid, [visit the Ordnance Survey website](https://www.ordnancesurvey.co.uk/).


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