How to construct a grid reference

A wildlife record is only useful if we know where the plant or animal was seen, and grid references are the best way of doing this. But why do we need to use grid references? Surely a place name will identify the location on the map? Well in many cases it can help, but grid references allow us to be more accurate. And, if you’re in a very rural area, the nearest town, village or settlement may be some distance from where you made your sighting.

If you don't know the grid reference, you can find it using

Alternatively, you can use any Ordnance Survey map to work out a grid reference. It is easier to work out an accurate grid reference using a 1:25,000 (4cm to 1 km / 2 ½ inches to 1 mile) Explorer map (the ones with the orange cover) rather than a 1:50,000 (2cm to 1km / 1 ¼ inches to 1 mile) Landranger map (pink cover).

The UK is divided into 100km squares by the Ordnance Survey and each 100km square has a two letter code (see map). This code forms the first part of your grid reference.

In Oxfordshire and Berkshire the codes are either SU (Berkshire and south Oxfordshire) or SP (north Oxfordshire) with a small part of TQ (eastern-most part of Berkshire).

Each 100km square is divided into 1km squares and these are the lines that you see on an Ordnance Survey map. Each one of these lines is labelled with two numbers. You can give a four figure grid reference by writing the 100km letter code (SP) followed by the number of the vertical line running down the left hand side of the 1km square (the easting) and then the number of the horizontal line running along the bottom of the 1km square (the northing).

To work out a six figure grid reference you need to sub-divide a 1km square into 100m squares – these sub-divisions are marked on the outside edge of a 1:25,000 Explorer map, but they are not numbered. You need to count how many subdivisions to the east of the nearest vertical 1km line and how many 100m sub-divisions north of the nearest horizontal 1km line your spot is e.g. 4 sub-divisions to the east and 2 sub-divisions to the north. These two numbers (4 and 2) are written as the third and sixth numbers in your six figure grid reference.

Common mistakes with grid references

Even people who are very used to grid references occasionally get them ‘back to front’ – this is when you give the northing before the easting instead of correctly giving the easting before the northing. One way to remember which comes first is to remember ‘Along the corridor and up the stairs’ – reminding you to give the figure along from the vertical line and up from the horizontal.

Another common mistake is to give the wrong 100km two letter reference (SP instead of SU or vice versa). In Oxfordshire and Berkshire it is particularly easy to remember that SU is to the south and SP is to the north by remembering that the ‘U’ in SU suggests ‘underneath’.