For those who are not familiar with the Ordinance Survey system
of detailing maps, we have included this section to help you
understand which grid square you are looking at and why it has been
named in this way.
For
the first letter the grid is divided into squares of size 500 km by 500
km. There are four of these which contain significant land area within
Great Britain: S,T,N, and H. (The "O" square contains a tiny area of
the North Riding of Yorkshire, almost all of which lies below mean high tide: OV000000)
For
the second letter, each large square is subdivided into 25 squares of
size 100 km by 100 km, each with a letter code from A to Z (omitting I)
starting with A in the north-west corner to Z in the south-east corner.
The accompanying map shows the resultant grid, with the squares
containing land lettered.
How to plot a National Grid Reference.
Plotting a grid reference of finds is essential to maintaining the integrity of their context.
Great Britain has been laid out on a grid system by the Ordnance Survey, and these squares
measure 100 Km by 100 Km. To make it easy to identify the squares, they have each been assigned a two letter code.
This forms the PREFIX of the National Grid Reference.
These grid squares can then be broken down into more squares, of 10 km by 10 km.
The basic premise is that the square is broken down into 10 segments, numbered from 0 – 9
from the South – West corner horizontally and vertically.
The reading from the bottom left hand corner of the square, to the bottom right forms the EASTING.
The reading from the bottom left hand corner of the square to the top left hand corner forms the
NORTHING.
Thus for National Grid Reference SP85
To get to this point on the map, you will need to follow the following rule:
"Along the corridor And up the stairs"
Therefore, to reach SP85, one goes across to square 8 on the bottom row, and up to square 5 on the vertical row. This produces a National Grid Ref (NGR) of SP85.
This NGR grid reference has an accuracy of 10 km, however, the accuracy can be increased by using a 4 figure grid reference that will produce an accuracy of 1 km.
Accuracy to 1km, is useful, however, in a real life situation, one obviously needs a greater degree of detail. If you get a grid reference which is 1km meters away from a point you previously discovered an object, then to paraphrase Sallah from Raiders of the Lost Ark, “You must be digging in the wrong place!”
Following the same methodology as demonstrated:
The prefix for the NGR is: SP
The Easting for the NGR is: 875
The Northing for the NGR is: 585
Thus the NGR in full is SP875585
Therefore, if you wanted to increase the accuracy of your Grid references, you would need to repeat
the above, but on a smaller grid square.
The levels of accuracy are shown below:
Number of figures Example Accuracy
0 SP 100 kilometer
2 SP85 10 kilometer
4 SP8252 1 kilometer
6 SP825525 100 meters
8 SP82555255 10 meters
10 SP8255552555 1 meter
12 SP825555525555 10 centimeter
For further reading on Grid References, please see the Ordnance Survey website –
http://www.ordnancesurvey.co.uk
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