Today we here at Ordnance Survey would like to go through understanding grid references.

Having selected the right Ordnance Survey map for our chosen leisure activty and understood what the map is telling us - it is now time to move on to looking at  grid references. When out you may have fallen and need the emergency services to come and find you or perhaps you've found a feature that you'd like to show to a friend - either way by providing them with a 6 figure grid reference you'll be able to accurately pin point the location.

This is our step by step guide on how to take a grid reference. To do it we'll be using this extract of OS Explorer Mapping.

Regardless of whether you are using an OS Landranger Map (the pink one) or, like this an OS Explorer Map (the orange one) you will notice that across the map there are thin blue lines making a grid of squares across the map. Each of these boxes, regardless of map type, is 1km across. At either end of the line, and usually somewhere in the middle of it too, is a number - this is the number we will be using for the grid reference.

The lines that run from top to bottom of the map are called Eastings - this is because the number of the line gets higher the further east you go. The lines that run from left to right horizontally across the map are called Northings - this is because the number gets higher the further north you go. It doesn't really matter what the lines are called - what does matter is the order in which you take them for the grid reference - the Easting comes first followed by the Northing.

Let's first take a four figure grid reference - this will pin point us in a 1km square box. We want to take the reading from the bottom left hand corner of the grid square that we are in. So imagining we are at Stonehenge - taking the easting first we would be in 1242.

That has placed us within a 1km square box and if we want someone to find us in a hurry - like the emergency services for example, we want to give them a smaller area to search. This is where we provide them with a six figure grid reference which will place us in a 100m square box.

This is where our imagination needs to come in. We need to look at the square 1242 and imagine that it is browkn down into smaller squares - 10 squares across x 10 squares high. Starting with 0 in the first box we need to count across which box we are in and add that number to our easting number. We then need to do the same with the northing number, counting up from the bottom (with 0 again being the first box). Putting this altogether provides us with a six figure grid reference - so in this case it would be 122421.

There is one further piece of information you will need to add to this - that is where you are on the national grid. The whole of Great Britain has been split into 100km square boxes and each box has a prefix of two letters. Without these two letters the six figure grid reference of 122421 could be in any one of the boxes. If you look on your Ordnance Survey map you notice in each of the four corners of the map there are to letters in blue - these are the prefix letters for the square you are in. So in the case of Stonehenge the full six-figure grid reference would be SU122421.

Some of the Ordnance Survey maps cover more than one national grid square - but where they meet the blue letters will mark what the other prefix letters are. If you find yourself in one of these locations -  The New Forest is an example of such a place - you should double check that you are giving the correct two letter prefix.

So that's how to take a four figure and a six figure grid reference, including the all importand national grid. Simon King has made a short film for Ordnance Survey on understanding grid references.