8 November 2012, 11:33
Mag north is always moving, always has look at the top of every Ordnance Survey Land Ranger map ever printed, along the edge you will see information on the issue of that map, what the magnetic variation is tween mag and grid north at that time, then use the formula to correct your route bearing. Grid to mag add, mag to grid get rid. EG my local map reports 4 degrees variation at time of print.