Understanding Magnetic Variation

If you have ever been taught how to take a compass bearing you might recognise the term “magnetic variation”.   Perhaps whilst you were in the Scouts or studying O-Level Geography you carefully learned, “Grid to Mag add, Mag to Grid get rid” or you used some other method to help you remember how to convert a compass bearing you took on your map to one you could use in the real world.  Unfortunately everything you learned is about to change.


First, lets go back to the beginning: what is magnetic variation?  Your compass contains a magnetic needle which points to Magnetic North. Magnetic North is located to the north of Hudson Bay in Canada and it is constantly moving, very slowly, eastwards. The grid lines on your map, however, point to Grid North. The difference between these two north points is known as Magnetic Variation. The size of the difference will vary depending on where you are on the globe and when you are there.  For example, in 2011 the magnetic variation in Lisbon, Portugal was 3 degrees West while in Moscow it was 10 degrees East.


Complex fluid motion in the outer core of the earth (the molten metallic region that lies 2800 to 5000km below the Earth’s surface) causes the magnetic field to change slowly with time. This movement is unpredictable so if your map is old the information on it may not be correct; it is therefore important to research the magnetic variation for the area you are visiting before you set off.  The British Geological Survey’s website is an excellent source of information.  If you visit their Grid Magnetic Angle Calculator page and type in your location using a grid reference or a postcode you will receive up-to-date results.   For example, if you type in a Keswick postcode the results are as follows:  “Magnetic north is estimated to be 1 deg 38 min west of grid north in July 2014”.


Once we have this information we can adjust any bearing taken using a map before we try to follow it over the ground.  In the Keswick example given above you would need to add 1 deg 38 min to any bearing taken from the map.  On most compasses each little black line on the compass housing represents 2 degrees.  As the magnetic variation in Keswick (and elsewhere in the Lake District) is currently less than two degrees it is virtually impossible to adjust it accurately.  If you want to try and adjust your compass by one degree then that would strictly speaking be more accurate but I would argue it would be better to spend your time practising following a bearing accurately because this is usually where error starts to creep in.  My advice to navigators on our courses over the last couple of years has therefore been to ignore the magnetic variation for the time being (because it is so small) and use the bearing they get from their map.


To make matters worse it is about to get even more confusing: for the first time since the late 1800s when Britain’s national mapping agency, Ordnance Survey, was created, magnetic north in the far south-west of England has shifted to the east of Grid North.  Until this happened anyone navigating in Great Britain could reply upon the fact that whatever the magnetic variation was in the area they were travelling they always had to add the variation on if converting from a bearing taken on a map to one they wanted to use in the landscape.  Once magnetic north shifts to the east of Grid North then the variation will need to be subtracted.   This means that all of the rhymes you learned in school will no longer be relevant.


For the next few years the magnetic variation in Great Britain is going to be very small.    If you happen to be navigating on the eastern outskirts of Penzance then you will be pleased to know that the Magnetic Variation in July 2014 is zero.  For all parts of the country the variation is going to be so slight that it will be possible to ignore it.  However if Magnetic North continues to move in the same direction we will eventually have to pay attention again and by then I hope I will have come up with a ditty to help navigators remember the new state of affairs.

Published 16/03/2016