# Coordinate reference system

The OS Open USRN product uses the British National Grid (BNG) coordinate reference system (CRS).

BNG uses the OSGB36 (EPSG 27700) geodetic datum and a single Transverse Mercator projection for the whole of Great Britain. Positions on this projection are described using Easting and Northing coordinates in units of metres. The BNG is a horizontal spatial reference system only; it does not specify a vertical (height) reference system.

Please view [A Guide to Coordinate Systems in Great Britain](https://docs.os.uk/more-than-maps/deep-dive/a-guide-to-coordinate-systems-in-great-britain) on our More than Maps platform for more information on CRSs.