Definition:

National Grid Reference

6-Figure References

Please read the guidance provided by Ordnance Survey on how to read a 6-figure grid reference, e.g. TL683365. This will give you a location in a 100 metres by 100 metres square.

8-Figure References

For some purposes, such as for supplying information to the Environment Agency, an accuracy of 10 metres is needed. So you will need to further sub-divide the 100 metre square into 10ths and that will give you the last figures for your grid reference, making an 8-figure grid reference:

TL68343651