Map reading Ordnance Survey navigation

‘Mag To Grid Get Rid & Grid To Mag Add' Or Is It?

Magnetic north is on the move again in Great Britain, if you navigate in the west of the UK be sure to read this first

Map reading Ordnance Survey navigation

The phrase 'Mag to grid, get rid & grid to mag, add' is a simple mnemonic to help you remember how to make the correct adjustment after either taking a bearing using a magnetic compass or taking a grid bearing from a map.

So If you’re a fan of this forces' favourite mnemonic to remind you how to make the correct calculations - by getting rid or adding the variation from magnetic north to grid north - it might no longer be as helpful as it used to be in some areas of the United Kingdom.

Particularly in the South West, the variation is now so small (0 Mils/degrees in Plymouth) that you’ll be able to effectively ignore the grid magnetic variation that changes each year. 

Expert military 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 Ordnance Survey (OS) or military maps). And if you’re operating in the west of Great Britain, there is a change to be aware of.

The red line represents the approximate path of where magnetic north currently equals the same as grid north.

Magnetic North Ordnance Survey
Magnetic north continues its march to the east [Photo: Copyright of Ordnance Survey]

In 2014, there was a significant change in direction of magnetic north relative to grid north on OS maps.

For the first time in Great Britain since the 1660's, magnetic north moved from being to the west of grid north to the east.

This change started in the very South West corner of Britain and affects the areas to the west of the red line but will continually shift east across the United Kingdom over the next 13 years.

Below is a rough guide as to when/where grid north and magnetic north will be at zero and no calculations will be necessary, unless you require the highest of accuracy.

Date of magnetic north changes

However, for those navigating further west of Plymouth will now have have to ditch the current mnemonic and come up with a new one.

What will you now use?

If map reading and using a compass is new to you, Ordnance Survey have some helpful guides to get you started.