How do Ordnance Survey grid references work?
The National Grid, defined by the Ordnance Survey, provides a unique reference system which can be applied to all Ordnance Survey maps of Great Britain at all scales. This includes both digital and paper mapping.
Using Ordnance Survey grid references we can uniquely identify a precise location anywhere in Great Britain. All OS digital and paper maps include all the necessary information to generate and locate grid references. Blue lines and numbers throughout the mapping highlight the national grid boundaries.
Understanding OS Zones
Great Britain is covered by 100 kilometre grid squares, each grid square is identified by two letters as illustrated. These grid squares are called zones. To quote a grid reference correctly you will need to know which zone you are in.
Grid references are quite often quoted without the zone. This is OK when referring to a specific map as the zones are much larger than a typical paper map and the reference will identify a unique position on that particular paper map.
When using digital mapping or GPS you will always need to know the zone as the areas of mapping are much larger covering multiple zones.
Using 1km Grid Squares
On Ordnance Survey maps, these squares are further divided into smaller squares by grid lines. For paper and digital mapping the smaller squares are 1km x 1km. These smaller squares are used to read grid references.
A grid reference consists of a zone, an easting and a northing. For example, in the illustration the grid reference of the red dot is TL 643 327. This means that the red dot (at the intersection of the red lines) is in zone TL and is 64.3km east and 32.7km north of the south west corner of the zone.
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