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Magnetic declination

“Mag to Grid get rid, Grid to Mag add” was a mnemonic used to help inexperienced navigators to remember to adjust their direction when navigating with both a map and compass.

The reason it is needed is that there is a difference between where the vertical lines your map point to (Grid North) and the direction your compass points (Magnetic North). This has worked, in the UK at least, since 1660’s.

It may be worth noting the difference between the three most commonly used “Norths”. Each one of these can be explained in great (and somewhat technical) detail, but the basics are:

  • Grid North: These are the blue vertical lines on an Ordinance Survey map. They run parallel to the line up the centre of the UTM time zone which you happen to be in.

  • Magnetic North: This is where your compass needle points to. From where you are Magnetic North follows local magnetic flux lines which are roughly aligned to a point at the top of the world at one end of the Earth’s Magnetic Field (where the magnetic field lines point straight down).
    For more information on “why” the Magnetic Pole move, you can read my blog here: >>>>>

  • True North: This is a line from where you are to the point at the top of the Earth’s spinning axis.

In the UK from the mid 17th centaury until a few years ago a compass would point to a position in Northern Canada, to the left (West) of where the vertical lines on a map point. The difference, between the two directions is known as the Magnetic Declination.

When navigating using both a compass and map it’s necessary adjust your direction of travel to compensate for the Declination.

Let’s imagine you point your compass at a hilltop and the bearing shown on your compass is 123 degrees. If you know where you are you can place your compass on a map, with the edge of the compass touching your location and, after rotating the base plate of the compass until the lines inside the dial are pointing towards the top of the map, the edge of the compass will point at the hill top (if your compass is long enough). In this case you used / gone from a Magnetic (compass) Bearing to a Grid (map) Bearing.

You can do the same in reverse. Put your compass on a map so the edge is touching your location and the hilltop (the arrow on the base plate needs to point in the direction you want to travel), then rotate the dial until the lines inside the dial match the vertical line on the maps. You can then read the compass bearing, from where you are to the hilltop. In this case you used / gone from Grid (map) Bearing to a Magnetic (compass) Bearing.

But don’t forget the lines on the map don’t point in the same direction as your compass, so you will need to adjust the bearing to compensate.

But all this changed (in the UK) in 2014 – as the Magnetic North Pole moved towards Siberia and so Magnetic North (see above) aligned with Grid North in the far South West of the UK.  As the Magnetic North Pole continually changes position, the direction a compass points to also changeds. 

The Magnetic North Pole is still moving in the same direction and the Magnetic North will eventually (taking around 20 years) slowly move right the way Eastwards across the UK.

At the moment (May 2021), depending on where you are in the UK, Grid North could be East or West of Magnetic North so you could still be saying “Mag to Grid get rid, Grid to Mag add” (even if the declination is only a fraction of a degree) or, for the those to the West of Magnetic North how about “Mag Unto, Grid Subtract”, hmmmmm! not very memorable is it.

Sooner or later someone will come up with something.