A grid reference is a way of defining a location within Great Britain.
It is usually an eight character item of the form

[2 Grid letters] [3 figures(easting)] [3 figures(northing)]

where the 2 Grid letters are the position in the picture on the right and the figures define a
location to within 100 metres.

The Ordnance Survey Grid Reference System lets you define any UK location
through the use of a map reference conforming to the following convention:

[2 Grid letters] [3 figures(easting)] [3 figures(northing)]

To arrive at this map reference, the United Kingdom is first divided into a series
of 100km squares.

Each 100km square is then given a unique two letter code such
as NT, SU, TQ, etc.

The approximate extent of each of these 100km squares is illustrated on the left.

Therefore the grid reference for any location in Wales always starts with one of the following:

SH, SJ, SM, SN, SO, SR, SS, ST

This indicates within which particular 100km square the location is to be found.

Much greater accuracy is of course required if the grid reference system is to be of any practical value. Each two letter 100km square is therefore subdivided by vertical and horizontal grid lines, at 10km intervals, to produce one hundred 10km squares.

The vertical line forming the left hand side of the 100km square is labelled zero.
Each successive vertical grid line within the 100km square is labelled in kilometres.

The horizontal line forming the bottom of the 100km square is also labelled zero.
Each successive horizontal grid line within the 100km square is labelled in kilometres.

Any intersection of a vertical and horizontal line within square SN is therefore so many
kilometres east of that squareâ€˜s zero point (the easting), and so many kilometres north of that
same zero point (the northing).

The centre point of square SN could therefore be said
to have a grid reference of SN 50 50.

Each 10km square within the SN square is then further
subdivided into a hundred 1km squares.

The diagram on the left is an enlarged view of the four 10km squares in the bottom left hand
corner of the SN square shown above.

The first 10km square is subdivided by vertical
and horizontal grid lines at 1km intervals.

The centre point of this 10km square could be said to have a grid reference
of SN 05 05.

To accurately establish the grid reference of the adjacent black dot
, a further 10 x 10 grid must be superimposed on the 1km square within which the dot is located.

The dot can then be seen to be 4.5km east of zero (easting), and 5.5km north
of zero (northing) within grid SN.

So its grid reference would be SN 045 055.

Explorer maps are amongst the most useful, for walking,
Ordnance Survey maps published.
They use a scale of 1:25 000 (4cm to 1km), and follow the general principal of the grid system
discussed above.

If only one 100km square is covered by a map, the two letter code
for that square can usually be found in each corner of the map. If more than one 100km square
is covered, the relevant two letter codes are usually clearly printed where the 100km squares
meet.

Each 10km square shown on a map is further subdivided into
one hundred 1 km squares. The squares are formed from blue grid lines, spaced 1 km apart.

The eastings are marked along the top and bottom edges of each map, and the northings
down the sides of each map.

When defining a location,
like the start of a walk, use the following convention for all OS Grid References:

[2 Grid letters] [3 figures(easting)] [3 figures(northing)]