Ordnance Survey – Great Britain's national mapping agency
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To recap: There is a point in Britain with full National Grid coordinates 538857.2345 metres easting, and 177345.3456 metres northing.
TQ 37 10 km by 10 km square
TQ 3877 1 km by 1 km square
TQ 388773 100m by 100m square
TQ 38857734 10m by 10m square
TQ 3885777345 1m by 1m square
The 100 km digits give you the National Grid reference prefix letters TQ. The more digits we follow this with, the more precisely we can can locate the position of the object of interest.
For use with leisure mapping, 'six figure' grid references are most popular: in this case, TQ 388773. This is the most precise reference we would usually use with Landranger or Explorer maps.