How to give a grid reference

The Ordnance Survey map references to glow worm sites are very important, as they enable us to locate its position to within 100 yards. If we want to check how numbers are varying in the future, this is essential – but private sites will not be visited without permission!

If you have an OS map, instructions on giving map references are shown in the margin. But basically, this is what you do. The map is divided into 1 km squares by grid lines numbered along the edges. You have to look along the bottom or top edges first and find the number of the square, then estimate the site’s exact position in tenths of a square. Then you do the same along the left or right edges. The numbers are also printed every so often on the map, which makes things easier:

The position shown has a six-figure reference of 513827. To complete the reference, you need the two letters which are given in a key somewhere at the bottom of the map, so the full reference might be ST513827 (Blewbury Down in Oxfordshire – I chose the numbers completely at random, and I have had no reports from there, but it would be a very good site to investigate anyway!).

If you don’t have an OS map of the area to hand, you can go to the Ordnance Survey website, Go to ‘Get-a-Map’, where you will be able to fill in the place name or postcode of the location and see a small-scale map of the area. Click on the big + sign to the left and you will get a more detailed section of the 1:50,000 map. You can also move around using the direction arrows to the left. Below the map you will see the grid reference of the centre of the map itself. Use the instructions above to work out the reference to your site, bearing in mind that the figures increase from left to right and from bottom to top.

TIP: If the OS website doesn’t recognise the place name you put in, try This has a much wider choice of place names, including woods, farms and hills, which for some reason are not given on the OS website itself. To get the grid reference of the centre of the map, click on ‘Click here to convert/measure map coordinates’ below the map and it will list the grid reference in different forms, of which the bottom set, LR, are the version we want.