HI Mate
Thanks for the kind words. As I say, I use it when delivering courses so if you have got the gist of it just from reading it without anyone talking you through it you are doing well!
The first thing about Mag Variation is it is quite advanced and it is perfectly appropriate to navigate without it in most eareas of the UK. It is used in "Micro-Nav" pinpoint navigation used in orienteering, Mountain Leader training and the army.
Regarding Magnetic Variation: Beneath the earth's solid crust is molten lava which is fluid and - because of it's high iron contect - magnetically charged. The iron particles align themselves N-S as they are free to move to some degree..all be it very s...l...o...w...l...y. It is the point somewhere towards the top of the earth that all compasses point because they too are following the arths magnetic alignment.
However...
Because the magma is fluidand the earth is spinning and other after effects of the formation of the universe all that time ago the location of the magnetic north pole does wonder about a bit under the earth's crust. This is very slow, and as it has been studies for quite some time we can predict in which direction it is moving and haw fast so we can account for it.
Remember, when an area of land is surveyed for a map it is only accurate on that day, not necessarily when it is printed and distributed and certainly not ten years down the road!
If you look at the legend of an OS map you will see a little bit about Magnetic Variation which tells you what you do. The info on magnetic variation differs on whichever map you are using depending on the map's area and when it was printed so check each time.
Lets look at an example:
The info in the legend says:
Magnetic north varies with place and time. The direction for the centre of the sheet is estimated at 3 degree 11 minutes (21 mils) west of grid north for July 2010.
Annual change is about 1/2 degree east.
So lets just say 3 degrees.
I take a bearing from my map (grid) and I look for the feature on the real world using the compass (mag) so I
add the 3 degrees to make my map relate to the real world a bit better.
If I find a feature on the horizon and I want to use it to see where I am on the map, I take a bearing to the feature using the compass (mag) and place it on the map (grid) and
get rid of the 3 degrees to make my bearing relate to my map.
That last bit...
Annual change is about 1/2 degree east. tells me the compass will point to magnetic north which is slightly west of grid north (that's the mag variation) but it is moving east; therefore next year the mag variation I have to use will be 2.5 degrees, not three. And in 6 years there will be no magnetic variation to worry about!
Hope this hasn't confused you further.
For a recent discussion on natural nav. Have a look
here.
