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Explanation of Ordinance Survey Coordinates
On this site the location of places will be described by using the
Ordinance Survey coordinate system.
The OS system is based on dividing land into squares each 100km by 100km.
Each square is given a code two letters long. Most of Wales is covered
by four squares:
SH:North-west Wales (Holyhead,Colwyn Bay,Caernarfon,Dolgellau)
SJ:North-east Wales (Wrexham,Mold)
SN:West Wales (Aberystwyth,Carmarthen)
SO:East Wales (Newtown,Brecon,Abergavenny)
There are also some small parts of Wales in other squares:
SM:St David's Head
SS:South Coast - Swansea to Llantwit Major
ST:Cardiff, Newport and lower parts of the valleys
The identity of the squares is indicated on OS and some other maps using
large blue hollow lettering. On the edges of the maps you should see
digits 0 to 9 corresponding to the horizontal and vertical lines. The most
usual system is to use six digits, the first three giving the E-W coordinate
and the second three giving the N-S coordinate.
Take the example of St Donats on the south coast between Swansea and
Cardiff, near Llantwit Major. The square is SS. It lies between 9 and 0
on the horizontal (E-W) scale and between 6 and 7 on the vertical (N-S)
scale. This means that the coordinates are ST9..6.. where the dots indicate
digist to be filled in.
On my map the the squares (10km by 10km) are 4cm, hence the scale is 2.5cm
to the km. St Donats is 1.45cm from the line to the left and 3.25cm from
the line below. The reamining coordinates are 1.45/4=.36 (horizontal) and
3.25/4=.81 vertical.
The full OS coordinates of St Donats are: ST936781.