Ordnance Survey FAQ and Excel Mapping Routines

Magnetic Variation:
Compass adjustment made easy


Every description I have seen of how to use a Silva type compass encourages the user to adjust his/her compass by adding or subtracting the magnetic variation after setting it from a map or after taking a bearing.  They come with helpful ditties to remember whether to add or subtract, but which may be invalid in other parts of the world.  The may be fine in a nice warm room and a steady table to rest on, but have you ever tried this in a howling blizzard?  You cannot do it with gloves on, but you are afraid to remove them because you know won’t get them back on over wet hands.  You cannot see the fine gradations on the scale because of the hail in your eyes (or you need your spectacles), you’ve forgotten whether to add or subtract, and you are shivering too much for careful adjustments anyway.   Oh the joys of hill walking in the UK!   I tried the method near that otherwise lovely Llyn y Fan Fawr (SS8321) and vowed to forget all that rubbish and devise a better way!!

The method I came up with is very simple and utterly reliable.   Note from the margins of your map where magnetic north is relative to grid north.  Now note where that is on the bezel around your compass needle capsule.  It might be 3 degrees west which corresponds to the bottom left of the N on my device.  You might be in far flung corners of the globe where it is 23 degrees east and that might correspond with a nick on the bezel to the right of north where you last fell on it.   If there isn’t an obvious mark then make one temporarily with a pencil or sticky paper until you trust the method enough to put a more permanent mark with nail varnish or whatever.  There are then just two VERY SIMPLE rules to follow in the field:

  1. When you have your compass placed on the map, set it according to the parallel north/south lines in the capsule, as usual, but do not then turn the dial  further to adjust for variation.
  2. When you are turning round or turning the compass to get the red end of the needle to point to the north mark, get it to point to your mark instead.

The method works whether you are setting your compass from the map in order to follow a bearing, or are taking a  bearing on a distant church to find where you are on the map.  It also avoids any ambiguity between rough bearings (“which road?   I’ll not adjust for variation”) and more precise ones (“is that faint track the one I want?”) in that both are the same.  It’s a method that’s been argued over and I’ve been told that it can’t be right for a variety of reasons – the least compelling of which was that “the military don’t use it”, but it is very effective.  It’s also the principle used for a ‘set and forget’ compass.  Give it a try and you’ll never look back.  From now on, only mugs will remember ditties like “Magnetic unto grid, subtract – MUGS” and continue applying minute adjustments to the dial in foul weather.

If you think about this principle of never adding/subtracting from the dial you’ll also get new perspectives on activities like taking a bearing on the point you’ve recently left (point white end of needle at the mark instead of deducting/adding 180) or ‘boxing’ round an obstacle (use E and W marks instead of adding/subtracting 90).

Phil Brady © 2012