How to identify a Tetrad

All Ordnance Survey maps have a grid of lines running from top to bottom and across the page, each of which is numbered. The lines make a pattern of 1 kilometre squares (1 x 1 kilometre). Every 10th line is slightly thicker and these 10km squares (10 x 10 kilometres) contain 100 1km squares and takes its identity from the numbers on the left hand side and the bottom of the 10km.

A Tetrad is 2km by 2km and contains 4 x 1km squares so it follows that the tetrads are bounded by the even numbered lines and there are 25 tetrads within each 10km square and each is identified by a letter as illustrated below 

If you have the OS map of Shrewsbury  (Sheet 126 in the Landranger Series) and find Wem (on the top half of the map) you will see that most of the town is contained by the even numbered lines 50 to 52 and 28 to 30. This means that the 10 km square is numbered 52 (5 from 50 and 2 from 28) and the tetrad is numbered 52E (0 from the 50 and 8 from 28). Clearly these numbers will be repeated on other maps in the series so each 100km by 100km square is identified by a couple of letters and the letters for this map appear in each corner of the map, that is SJ. Therefore, the tetrad that covers most of Wem is SJ52E.

If you have the OS map of Ludlow & Wenlock Edge (Sheet 137 in the Landranger Series) and find Craven Arms (on the bottom half of the map) you will see that most of the town is contained by the even numbered lines 42 to 44 and 82 to 84. This means that the 10 km square is numbered 48 (4 from 42 and 8 from 82) and the tetrad is numbered 48G (2 from the 42 and 2 from 82). Clearly these numbers will be repeated on other maps in the series so each 100km by 100km square is identified by a couple of letters and the letters for this map appear in each corner of the map, that is SO. Therefore, the tetrad that covers most of Craven Arms is SO48G.

Remember that in order to get the number sequence the right way round always check the vertical line first and then the horizontal line next.

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