The National Grid reference convention

The map projection used on OS Great Britain maps is known as the National Grid. The TM eastings and northings axes are given a ‘false origin’ just south west of the Scilly Isles to ensure that all coordinates in Britain are positive. The false origin is 400 km west and 100 km north of the ‘true origin’ on the central meridian at 49°N 2°W.

To reduce the number of figures needed to give a National Grid reference, the grid is divided into 100 km squares, which each have a two-letter code. National Grid positions can be given with this code followed by an easting between 0 and 100 000 m and a northing between 0 and 100 000 m.

The National Grid, showing the true origin (t.o.) and false origin (f.o.).