The ability to map read, even to a basic level, is very useful when walking on Dartmoor, and absolutely essential if you are heading into the more remote areas. Dartmoor walkers will therefore be interested to hear that magnentic north, as opposed to “Grid North” that is shown by the lines on your map, is in the process of going walkabout and is going to affect the south-west before the rest of the country.
The change is gradually sweeping across the globe. In Britain it has reached Cornwall first and magnetic north is currently moving around 30km per 6 months, so the landmark moment (in our geographic era) of magnetic north being east of grid north will eventually affect the whole of Great Britain.
For more information on this please follow this link to the Ordanance survey blog.