We should all know there's a difference between magnetic north, the direction your compass points, and the grid lines on a map. In Britain we've always been able to correct for this in the same way - but that's no longer true. If you're walking in the West Country, West Wales or the Hebrides, then this is something you may need to know. The experts explain in this article, re-published from the Ordnance Survey blog.

As expert map readers will know, when you're out and about navigating with a compass, there is a difference between magnetic north (where the compass points) and grid north (the vertical blue grid lines shown on OS maps). And if you're exploring in the west of Great Britain, there is a change to be aware of…

The difference between magnetic north and grid north is often referred to as grid magnetic angle and it not only varies from place to place, but changes with time too, and needs to be taken into account when navigating with a map and compass.

In 2014 there was a significant event in the changing direction of magnetic north relative to grid north on OS maps. For the first time in Great Britain since the 1660s, magnetic north moved from being to the west of grid north to the east. The change started in the very south west corner of Britain, and currently affects the areas to the west of the line on our map; it will slowly progress across the whole country over the next 12 to 13 years.

Five years on, we thought it was time for an update, so used the excellent British Geological Survey blog on the changing direction and their grid magnetic angle calculator to estimate the region where magnetic north is now east of grid north and this is what is shown on the map to the west of the red line. The line represents the approximate path of where magnetic north currently equals grid north. We also checked the current angle between magnetic and grid north for a selection of locations across Britain and estimated when the difference between magnetic and grid north will reach zero.

Add for mag, rid for grid

We show magnetic north on all of our maps (and state the date it was calculated), but you can always visit the BGS site for the latest information. New maps in affected areas will also have a new icon we have created in the legend to show the new relationship between the three Norths (magnetic, grid and true).

And if you're a fan of the mnemonic ADD for mag, RID for grid to help you remember how to make your bearing calculations…that will no longer be helpful in some areas of the country. In the south west, variation is now so small (0 degrees in Plymouth) that you'll be able to effectively ignore it, but that will gradually change – so does anyone have a suggestion as to what will the new mnemonic be...?