National Grid References
Ordnance Survey has divided the United Kingdom into 100km squares, which are each given a two letter code. East Sussex falls within the 100km grid squares of TQ, TV and TR. West Sussex is covered by the squares TQ, SU and SZ.
The digits of a grid reference identify a location within this 100km square. The first half gives the distance East from the western edge of the 100km square. The second half gives the distance North from the southern edge of the 100km square.
To give a grid reference, first read the two-figure number across the bottom of the map (west to east) and then the two-figure number at the side of the map (south to north). This four-figure grid reference will give the actual kilometre square.
This is a large area and a more accurate reference is obtained by sub dividing the kilometre square further to a six-figure reference. This is gained from using ‘imaginary’ grid lines, further dividing the grid square into 10 from ‘west to east’ and then from ‘south to north’.
The “DINTY” Tetrad System
The DINTY system breaks up a 10km × 10km OS grid square in to 2 × 2km sub-units, known as a tetrad. The 10km ID (e.g. TQ02) is obtained by using the first digit of the x axis then the first digit of the y axis. The DINTY Id can be obtained using the table below.
This system is a system that is often used in species recording as sometimes a recorder may wish to keep the location of a record cryptic, but finds that a 1km grid reference is too revealing.