Using Magnetic Variation
- Locate the information on your map that tells you about the local magnetic variation. This can be found in the Key and also on the map itself.
For the time being let us learn how to use the information on the key.
'Different' Norths On a typical Landranger type map the information will look like this:
Difference of true north from grid north at sheet corners
NW corner - 1° 20' (24 mils) E
NE corner - 0° 54' (16 mils) E
SW corner - 1° 19' (23 mils) E
SE corner - 0° 53' (16 mils) E
Magnetic north varies with place and time. The direction for the centre of the sheet was about 4 ½° (80 mils) W of grid north in 1990 decreasing by about ½ ° (9 mils) in the next three years.
To plot the average direction of magnetic north join the point circled on the south edge of the sheet to the point on the protractor scale on the north edge at the angle estimated for the current year.
Note the last paragraph. This will be explained later.
- To compensate for the angular difference take note of the average variation for the sheet (this is more than acceptable for any sensible work. If you really want to be perfect, take note of the difference between true north and grid north too). In our example sheet above this would give us a value of around 4° (4 ½° - ½° as we are long past 1990). (Note: A difference of 4°, if you walked in a straight line for 10 km would mean you would be around 700m away from your intended destination. So for most everyday uses of bearings the difference is negligible over sensible distances)
- Measure your bearing as you would normally, ignoring magnetic variation.
- When you have your bearing adjust the value by 4°. In our case we would have to ADD 4° to our bearing value to get a 'true-to-life' direction. If the diagram indicated that grid north lay to the left of magnetic north, you would have to subtract the angle from the bearing.
- You can now follow your compass bearing safe in the knowledge that you will be heading in the correct direction.