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The system used in Britain for calculating a position is known as the National Grid. The map on the left shows the country divided into 100 km grid squares. As well as being able to cross reference the position of a square by the distance east and north of the base line, each grid square has a unique two letter reference. Each 100 km grid square is sub-divided into 1 km grid squares which is how it appears on the Ordnance Survey maps. |
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In the sample map shown left, the 1 km grid square reference position is 14 50. (14 east, 50 north).
This reference can apply to any of the 100 km grid squares, so it is necessary to identify which 100 km square our 14 50 would be found in. There are two commonly used methods, either 414 350, which refers to the total number of kilometres east and then north of the base line. This is the method adopted by most internet based map web sites. The other method is SK 14 50, this identifies which 100 km grid our 1 km square is to be found in. If you look closely at the right hand corner of the sample map, you will see that the Ordnance Survey cater for both methods. A one kilometre square is still a big area, you can fit a lot in it. For more accurate naviagtion and positioning the 1 km square can be further sub-divided into 100 metre, then 10 metre and 1 metre squares. On the sample map, the red square over the Izaak Walton Hotel represents a 100 metre square area. Using the graduated scale that is found on any O S map it is easy to calculate the grid reference of the Hotel as 4143 3508 (using the kilometric system, 414.3 km east, and 350.8 north of base line) or SK 143508 (100 km grid square SK, 14.3 kilometres east and 50.8 north.) The latter method is the one most commonly used in walking books etc and also note that the 3 digit easting and 3 digit northing are shown together without a space. |
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My own preference and one used throughout this site is for the Kilometric system, with grid references being shown down to the one metre level. In practice though the last digit is usually either a zero or a five. On the sample map shown left, the position of the junction of the two footpaths north west of the hotel is GR (this is for Grid Reference) 414330 (414 kilometres, 330 metres east of base line) 350880 (350 kilometres 880 metres north of base line). This would be written down as GR 414330 350880. |
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A useful aid to calculating a grid reference, particularly if you are on top of a windy fell is a Romer. This is a small piece of clear plastic with various vernier scales on. They are very cheap and available from all good walking shops. Some of the more expensive compasses also have have them on.The sample map shown left gives an idea of how they work. If you know where you are on the map and want to work out the grid ref then place the romer grid on the respective 1 km square and the edges of that square intersect with the scale on the romer indicating your position. For example, the red arrow is at position GR 414875 350775. The reverse is also true. If you have a reading on your GPS, then by lining up the scale on the left hand side and bottom of the respective 1 km square you can use the romer to show your precise position on a map.
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Centre Of Great Britain One of the most precise and popular grid references is that which refers to the most central point in Great Britain. According to the O S website it is SD 64188.30 56541.43 or GR 364188.30 456541.43. This give the position down to one centimetre. Those of you wanting to look it up on the map will need OS explorer map OL41, (Forest of Bowland and Ribblesdale). |
