Do you need to worry about Magnetic Variation?

If you look at your OS map you will see a little diagram which looks like this. It indicates the direction, although not the actual angles, by which the three norths vary. So what's this all about? Let's start with some definitions.

• True North - is the direction of the actual North Pole, the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface¹.
• Grid North - on Ordnance Survey maps this is the direction of the vertical grid lines on the map. The OS grid is a rectilinear grid: that is to say that each grid square is 1km on a side so it's essentially assuming that the world is flat! The consequence of this is that grid north varies a little from true north across the UK (Northern Ireland, who have their own mapping agency, use the same grid).
• Magnetic North - the direction your compass points towards. This varies because the magnetic north pole moves about (something which I may come back to in another article: it's an interesting, and quite scary, story).

The difference between grid and magnetic north is what OS call the "Grid Magnetic Angle" or what most people call the "variation" you need to worry about when using OS maps and if you're old, like me, you would have been taught that when converting a bearing from map to compass and vice versa you had to add or subtract the variation to ensure you didn't get errors.

Now if you look at the top of your OS map you will find a statement about the variation between grid and magnetic north on the particular map that you are using. So here's the text from a Landranger taken at random from the shelf.

I learned to use a map and compass as a scout in the 1970s and in those days the variation was around 8° but I've got old maps of my Dad's which show variation of around 15° and I was told then that this was because of the wanderings of the magnetic north pole.

As you can see from that statement above it's still going down. There's 60' to a degree so it's moving east at about 1° every four years. At this rate by about 2022 there's going to be practically no variation, on this map at least.

And this has already happened on some maps. The Ordnance Survey page Calculating magnetic variation says of the Penzance sheet:

The Magnetic North to Grid North is predicted to be 0° 00' west of grid north at the centre of the sheet in July 2014, with an estimated rate of change of 11' east a year.

Even for Lowestoft, which is as far east as you can get and so has the greatest variation, and lies on the Norwich & The Broads, Great Yarmouth sheet they say:

Magnetic North to Grid North is predicted to be 3° 18' west of grid north at the centre of the sheet in July 2014, with an estimated rate of change of 10' east a year.

The reason for this can be seen in the diagram on the right. the UK sits at about -8° to +2° longitude so the magnetic north pole is currently moving into a position on the opposite side of the true north pole to us, and true north isn't that far away from grid north.

The practical upshot of all of this is that the variation between grid and magnetic north is really very little, less than 1% across the UK, and I defy anyone using a typical Silva type compass to be able to take bearings with that degree of accuracy, or indeed walk a bearing with that accuracy.

So for the next decade or so I would suggest you can stop worrying about variation and what you have to add or subtract as it doesn't really make enough difference to matter for most practical purposes, certainly when out walking.

1. Well in theory at least. It turns out that the Earth wobbles on its axis a bit, but let's not worry too much about that.

Tagged: maps

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