By: Blonde Two
All good navigators know that magnetic north (where your compass points) is not usually the same direction as true north (direct line of longitude to the north pole). This means that grid north (the northy lines on a map) don’t match up exactly with what your compass says. To make things more complicated, magnetic north (mag) is a tricky creature to pin down and likes to move around.
When you are lost in the middle of a dark moor (and indeed, at most other times) it is difficult to remember what all of these “north words” mean. Most good navigators therefore, also know that when you take a bearing you add a couple of degrees (this varies according to your map but I just nudge mine a bit) if you are going from your map to the real world and take away a nudge of degrees if you are going from the real world to your map. Hence the saying “mag (magnetic north) to grid (map north) = get rid.”
Confused – me too, but doing it is a lot easier than explaining it. Anyway, things are about to get even trickier because naughty old magnetic north is on the move and has decided that being the other (west) side of true north is much more de-rigueur than that boring old east side.
The Ordnance Survey explain it here; http://www.ordnancesurvey.co.uk/blog/2014/01/magnetic-north-is-on-the-move-again/
I have no idea if I explained that right or not but I do know that we are going to need some new rhymes to help us to remember …