Ronnie Irvine/Elaine Simpson 2017
 Magnetic north is not constant and varies from place to place.   In Northern Ireland magnetic north is about 10° west of grid north.  This is known as magnitude variation and is not constant.  It varies from place to place and slowly changes with time.  Details of magnitude variation, annual rate of change and date of calculation are printed on maps. The difference between grid and magnetic north must be considered when converting between bearings on a map and bearings on the ground. The most common use of a compass is to identify the direction in which you should travel to reach some point which  cannot be seen to due to poor visibility or the shape of the ground.  Working out a direction of travel. 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. Try this. In the following exercise use the 1: 25,000 Mournes Map which is easier to work with. Find Slieve Donard and Crossone. Assume you are on top of Donard and wish to use a compass to reach Crossone. Hold the map flat 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
Using a compass.
A compass indicates the direction of magnetic north and can be used to measure directions relative to magnetic north.  Hill-walkers can use a compass for a wide variety of reasons.  Indicating the direction of an unseen object To identify unknown peaks And to accurately fixed location The most common type of compass is the Silva-as used in the diagrams.  Remember the three different types of north.  True north-direction of the North Pole Grid north-which is map north Magnetic north-where the compass needle points.  
Now rotate the compass dial 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-which does not take magnetic variation into consideration. This variation must be added to the grid bearing already calculated. Magnetic variation in the Mournes is about 8° . This variation must be added to the grid bearing already calculated. The magnetic bearing therefore is 114°. Correct in 2010-adjust for 2017 Rotate the dial to the new bearing-this is the reading shown in the photograph immediately above.  
You want to follow the bearing?   Hold the compass flat on your hand.  Rotate the whole compass until the north needle (red) is directly over the north arrow on the base plate of the dial. The direction of travel indicator indicates the direction of Crossone from the top of Slieve Donard.