Grid References

Creating a Grid Reference

Even if you are using a GPS you will need to be familiar with how to create a grid reference and how to transfer a grid reference from your GPS back to a map.

Basic Map Skills showed how to use some of the key features of a map. In this section we will concentrate on using the grid lines (typically in blue or black) on a map in order to take a grid reference for a landmark.

For this example I will use the Trig Point on Fan Fawr (blue dotted triangle on map below).

The first thing that we need to do is to find the Alpha-grid reference from the map. On this particular map it is printed in several places over the face of the map in large hollow blue letters. It is 'SN'.

The second thing that we do is find the Eastings (X) and Northings (Y) for the grid square that we want. It is not shown on this picture but is in fact 96 E, 18 N.

A very rough but valid 4 figure grid reference would be 'SN 9618'. That might be enough for a helicopter to find you but is otherwise useless. We need to be much more accurate. For this we need to use the scale on the compass base plate (see Compass basics).

Overlay the compass onto the map so that the point of the 1:50,000 scale (the same as the map) is exactly on the centre of the trig point.

On the horizontal 1:50,000 scale you can see that the nearest mark to the vertical blue line the marks the left-most edge of the square containing the trig point is 5. On the vertical scale, the nearest point that crosses the lower horizontal grid line is 9.

If we modifiy our original grid reference to include these more accurate measurements then 'SN 9618' will become 'SN 965189'. Each grid square on this map is 1Km and by sub-dividing it with the compass into a 10x10 grid we have a grid reference that is accurate to within 100m. 'SN 965189' is called a '6-digit grid reference'. This may sound poor when compared with a GPS but is adequate mostly. With a little bit of practice it is possible to estimate an 8-digit grid reference using the same compass scale. From the above picture an estimate at an 8-digit grid reference would be 'SN 96481892'.

GPS Grid References

It is now worth talking about GPS Grid References.

When a GPS shows a grid reference on its screen it will show an 8 or 10 digit grid reference. So for example, if I was standing on the Trig Point my current GPS would give me a 10 digit grid reference that would look like 'SN 96482 18914'.

In theory a 10 digit grid reference will give you a location which is accurate to within 1 metre. This may sound fantastic but in practice your GPS may not be able to see sufficient satellites to justify such an accuracy. In which case, one or two of the least significant digits should be ignored.

On my GPS the satellite page also shows the accuracy of the readings. This is critical information since it tells you how much faith to place in your GPS. In open countryside the accuracy is typically stated at around 10m.

In practice over the years I have found that various modern GPSes seem to be reliably accurate to 10m. So, whenever I look at my GPSes grid reference I take into account that it is typically accurate to 10m (and can verify this if I need to on the satellite page) and read the 10-digit grid reference that it gave me of 'SN 96482 18914' as an 8 digit reference of 'SN 9648 1891' which is still more accurate than I could achieve using a map of this scale (1:50,000).

Note: When working on a single map it is common practice not to worry about the alpha prefix.

Wot no Compass?

If you are one of an increasing number who just carry a GPS and do not carry a compass then you will need to consider carrying a separate map Scale template that can be bought from many map shops.

If you do insist on not carrying a compass then it is well worth considering a GPS that also includes an electronic compass. This will be less accurate than a traditional compass but will generally be adequate since the GPS will only use it when it loses satellite tracking.

If you cannot get hold of a scale then at a push you can use a ruler. On a 1:50,000 map 2mm = 100m and on a 1:25,000 map 4mm=100m.