To do this conversion you should first look for the north diagramm that can be found either on the sides or in the key of a map. e.g. :
The magnetic declination on a map corresponds to its center in january 1979.
It decreases each year of 0.07gr (0°4’).
You should now calculate the magnetic declination.
In that case on january 1st 2001 the declination had decreased of :
4’ x 22 years = 88’ = 1°28’
It is worth : 5°45’ - 1°28’ = 4°17’
On a compass a precision of 4° will be quite enough. (for more details see conversions.
You then need to remember the definition of a walking angle :
- The magnetic direction MD is the angle between the magnetic north and the direction you want to follow
- The geographical direction GD is the angle between the geographical north and the direction you want to go to. To switch from one angle to another see the picture on the side.
Be Careful ! The magnetic declination will change in time but also in space (the above facts are true for France) You should calculate the new declination each time you change of map.
