Map References
A full reference which is unique across Britain will consist of two letters followed by (typically) six digits giving resolution to 100 metres eg (SS 599 939). Eight digits are needed for 10 metre resolution and 10 digits for one metre. The two letters indicate the 100Km by 100Km square in which the point lies and are often omitted since this is usually unambiguous.
The first letter gives the 500Km by 500Km square. 25 different letters (omitting I) could be given here though only five actually map to the covered land areas: S for south west, T for south east, N for the north and Scotland and H for the Shetland Islands. The “O” square contains a tiny area of the North Riding of Yorkshire near point OV000000, but it looked pretty inaccessible when I paid a pilgrimage there! See http://www.geograph.org.uk/article/In-Search-of-OV0000 for a description by Peter Standing. The second letter gives the 100Km by 100Km square within that and is laid out with A in the North Westcorner, Z in the South East thus:
ABCDE
FGHJK
LMNOP
QRSTU
VWXYZ
See http://www.ordnancesurvey.co.uk/oswebsite/gi/nationalgrid/nghelp1.html for a map of the squares and a good tutorial on references.
The digits should be split into two groups – in the above example of SS 599 939 the point is 59.9 Km east and 93.9 Km north of the south west corner of square SS.
The measurement of a reference is made easier by the regular grid of squares representing 1Km (or 10Km for motoring maps) on the ground and printed on every map. The references of those grid lines are shown on the margins of the map and, on some, at intervals across the map. To determine a reference on a map with a 1Km grid:
- note the letters square from a description in the margins of the map or overprinted in the corners (eg SS)
- read off the two digit eastings number at the bottom or top of the map against the vertical line to the left of the point (eg 59)
- estimate the horizontal distance in tenths from the line (eg 9)
- read off the two digit northings number against the side of the map (eg 93)
- estimate tenths north from this (9)
The resulting reference SS 599939 resolves to 100 metres and the procedure is described on the margin of every OS map. If you wish you can use a centimetre ruler (there is one on my compass) to help you achieve more accuracy or you could invest a pound and buy a plastic scale called a romer. Be warned though that a romer is transparent and easily lost – mine vanished a week after I purchased it, re-surfaced briefly after a year and has not been seen since! Note that dividers or special scales are not needed as I understand they are with some US maps.
The margins of the maps also have ‘ticks’ and the body of the map has blue crosses to show latitude and longitude. Note that these are relative to the OS 1936 datum and not WGS84 which is becoming increasingly standard across the world. Also note that the lines of latitude and longitude are both curved and are not parallel to the printed OS grid.