Grid Lines/Grid (Kilometre) Squares/Map References
On each Ordnance Survey 1:50000 series map there is a series of
intersecting blue Grid Lines; one set runs vertically up and down the
map and are known as Eastings, the
other horizontally across the map and are known as Northings. Grid Lines are often abbreviated as GL.
The Grid Line after 90 is 00 and then the sequence repeats. Each 100
square kilometre area bounded by Eastings of 00 and Northings of 00 has a
unique two-letter code. This code is shown in each corner of the map and
astride any 00 line at the edge of the map.
Check this link for the OS 1:50000 Maps coverage of the UK.
Note that the
horizontal reference is quoted first. An easy way to remember the order is
the phrase "Along the corridor (travel horizontally) then up the
stairs (travel vertically)."
A Grid Square defines an area of one kilometre by one
510210 represents the bottom left corner of
our sample square; 510220 the top left; 520210 the bottom right; 520220
the top right; and 515215 the centre of the Grid Square. Note that such
references do not actually refer to a point but to a mini-square of 100
metres by 100 metres whose origin is given by the 6-figure reference.
However, it is common in rally navigation that a 6-digit reference
actually refers to the origin itself; although qualification is often by
quoting fractions of tenths (¼, ½, ¾) e.g. 510½214½.