## Summary

**a grid reference...**
- is used to plan routes and to pass to other people so that they know where you are, especially in an emergency situation

- normally quoted as a 6 digit number, with prefix of two letters

- the two letters are important, as each 6 digit grid reference repeats itself every 100km

### Calculating a Grid Reference in Summary

- read numbers from the map first from the horizontal - finding the vertical line that passes the left of the grid square containing your target location. These are the first two digits of the grid reference

- then secondly from the vertical - finding the vertical line that passes the left of the grid square containing your target location. These are the 4th and 5th digits of the grid reference

- identify the target grid square - this is the 1km grid square

- sub divide the 1km grid square into 10 by 10 smaller squares - each small square representing 100m by 100m - by estimating or using a roamer scale on a compass base plate

- 100m on a 1:50,000 scale map is 2mm and is 4mm on a 1:25,000 scale map

- work out how many small squares across - this is the 3rd digit of the first set of two numbers

- work out how many small squares up - this is the 3rd digit of the second set of two numbers

- finally, add the national grid prefix - two letters - which can be found in the upper corner of the map (or legend)

### Care!

Some maps, especially 1:25,000 scale, have more than one 100km by 100km grid represented on them and you need to make sure that pick the correct two letter prefix that relates to your grid reference. It's best to know before you set out whether that's an issue with your map, as the last thing you want to be doing is trying to open out the map in a strong wind to find out!

The classic example of 4 large (100km by 100km) grid squares being on the same map is in the Cairngorms near to Ben Macdui, so without much effort you could be in a position where you'd need to prefix grid references with 4 different pairs of letters in one day.

©Crown copyright and database rights Ordnance Survey licence number 100054073 2013