How to Use Grid References
Some of the most useful maps for understanding your family history are those produced by the Ordnance Survey.
All maps and plans published by the Ordnance Survey carry a system of grid lines, dividing the maps into squares. The grid lines are numbered and provide a single reference system for the whole of Great Britain.
Wales is covered by the eight squares shown below:
For the purposes of the rest of this explanation, we'll concentrate on the 100-kilometre square "SN" (right) which includes parts of the counties of Cardiganshire, Carmarthenshire, and Pembrokeshire in west Wales.
Each of the 100-kilometre squares is subdivided by vertical and horizontal grid lines into one hundred 10-kilometre squares. The diagram (right) shows square SN subdivided into 10-km squares.
The grid lines forming the 10-km squares are numbered in kilometres.
The full grid reference for the square is "SN1040". Adding the prefix "SN" to the grid reference tells us in which 100-km square the smaller square is situated.
Wales is covered by 19 maps in the OS Landranger series.
This is a scan of the top left corner of OS Landranger sheet 145. The corner is marked "SN" to remind us that we are in the 100-km square labelled SN.
This is a scan of the bottom right corner of OS Landranger sheet 145. The corner is marked "SN" to remind us that we are still in the 100-km square labelled SN (some sheets in the Landranger series cover areas from two or more different 100-km squares).
The picture below is a scan of another part of the same OS Landranger sheet 145, this time including the area surrounding the town of Cardigan (Aberteifi). I've marked in black the eastings and northings of the vertical and horizontal grid lines. Normally, with the map in front of you, you could check these from the markings on the edges of the sheet, as in the two previous scans. Notice that on the right, the northings are repeated in blue on the map itself to aid identification.
Most of the town of Cardigan is contained within the two 1-km squares whose grid references are SN1746 and SN1846. We could therefore quote either of these grid references to locate the town of Cardigan.
Using the letter combination (e.g. SN) plus four digits (e.g. 1746) enables us to locate or describe the location of a feature with an accuracy of 1 kilometre - but we can go one step further. In our imagination we can subdivide each 1-km square into one hundred smaller squares, each only 100 metres across. We can then define the location of a feature to within 100 metres.
For example, consider the feature enclosed in the small red circle. The name alongside indicates it is a property, probably a farm, called Pen-Lan. This feature is in the 1-km square whose grid reference is SN1946. However, its easting is actually between 19 and 20 - we might guess its easting is about 19.3 km. Similarly, its northing is actually between 46 and 47 - let's say about 46.7 km. We can therefore refine the grid reference and use six digits rather than four to improve accuracy. We now quote the grid reference of Pen-Lan farm buildings as SN 193 467, or SN193467.
With larger scale maps, it's possible to use 8-figure grid references, giving an accuracy of 10 metres.
Now find out more about Ordnance Survey maps.