Where is North?
True North is the direction to the North Pole, this can be ignored for hill-walking purposes (except possibly at night, with no equipment and with only the Pole Star to navigate by).
Grid North is Map North and is the direction to which the grid lines on a map point.
Magnetic North is the direction in which the compass needle points. In Northern Ireland Magnetic North is roughly 10° west of Grid North -this difference is known as the magnetic variation.
Magnetic variation is not constant.
It varies from place to place, and is slowly changing with time. We must take into account the difference between grid and magnetic north when converting between bearings on a map and bearings on the ground.
Working out a direction of travel.
The most common use of a compass is to identify a required direction of travel to reach a point, hidden because of either poor visibility or terrain.
The direction of travel is measured from the map, adjustment is made for magnetic variation and the compass used to indicate the direction of the object.
Use the 1 : 25,000 Mournes Map Which is easier to work with . Find Slieve Donard and Crossone and assume you are on top of Donard and wish to use a compass to reach Crossone.
1. Hold the map flat.
2. Place one of the long edges of the compass along the imaginary line Donard and Crossone on the map, making sure that the direction of travel arrow is pointing towards your destination -Crossone
3. Now rotate the dial of the compass so that North on the dial is pointing towards the North of the map by aligning the parallel lines on the base of the compass dial with the north/south grid lines on the map. Read off the bearing (green dot on the map) at the direction of travel indicator.
From Donard to Crossone the bearing should be 106°- this is the grid bearing.
4. Magnetic variation in the Mournes is about 8°-check this from map legend. This variation must be added to the grid bearing already calculated. The magnetic bearing is therefore 114°
5. Rotate the dial to the new bearing-green dot
6. Now the interesting bit-hold the compass flat and rotate the whole compass until North on the dial is pointing to Grid North on the map.
The direction of travel indicator shows the direction of Crossone from the top of Slieve Donard-green arrow superimposed in picture.
Off you go and don’t get lost .
Remember, accuracy is important. An error of 6° will put you 100m of course after walking 1km. In poor weather you might only see 10m !! Think of hitting a golf ball-a small angle of error in the clubface leads to a ball well off line.