THREE NORTHS
True, grid and magnetic
True north is set by the Earth's rotation. Grid north follows the map's grid lines. Magnetic north is where a compass points, and it moves slowly over time.
Check a cityStart
WHAT OS AND BGS SAY
Which way
is north?
In Britain there are three answers.
True north points to the North Pole, grid north runs up the lines on an Ordnance Survey map, and magnetic north is where your compass needle points. The angle between the last two keeps changing, and it matters when you turn a map bearing into a compass bearing.
CHECK THE DATEon your map's north
Check a cityKeep in mind
- Know your three norths
- Find the angle
- Check it is current
- Compass work in three lines
THE SHORT ANSWER
Three norths,
one angle to watch.
Map work uses grid north. Your compass uses magnetic north.
The gap between them is the grid magnetic angle.
THE ANGLE
Grid magnetic angle
Ordnance Survey and BGS call the angle between grid north and magnetic north the grid magnetic angle. It varies from place to place and year to year, so each map gives the date it applies to.
How to use it
MEASURING IT
Checked across Britain
The British Geological Survey measures the magnetic field at 41 repeat stations and three observatories, and updates a model every year that supplies north directions for OS maps.
See the numbersA QUICK CITY CHECK
How big was
the angle?
Pick a city to see the grid magnetic angle Ordnance Survey estimated in March 2019, and when it expected the angle to reach zero. The bar shows the size of the 2019 angle.
From Ordnance Survey's March 2019 estimates using the BGS calculator. Negative means magnetic north was west of grid north. Use the BGS calculator for today's value.
TWO NUMBERS WORTH KNOWING
A rare swing,
closely watched.
One date for how unusual the change is,
one number for how it is tracked.
1660s
Last time magnetic north was east of grid north in Britain
In 2014, magnetic north moved east of grid north in the far south-west of Britain for the first time since the 1660s, and the change is spreading across the country.
HISTORY · ORDNANCE SURVEY
41
Repeat stations across Great Britain and the Isle of Man
Each is visited every four years. Readings are combined with three continuously running observatories to update the UK magnetic field model every year.
MEASUREMENT · BGS
BGS predicted that true, grid and magnetic north would line up along the 2°W meridian between 2022 and 2026, moving north from Dorset towards Fraserburgh in Aberdeenshire.
FOUR STEPS WITH MAP AND COMPASS
Find the angle,
then apply it.
These steps follow Ordnance Survey's guidance on the three norths.
Step 1
Find the angle on your map
OS Landranger maps show it in the middle of the north margin and in the legend; OS Explorer maps give it in the legend.
Step 2
Check the date
The printed angle is only correct for the time of printing. For a current value, use the BGS grid magnetic angle calculator for your location.
Step 3
Know which side it is
Where magnetic north is west of grid north, the rhyme applies: from grid to mag, add; from mag to grid, get rid.
Step 4
Watch for the swap
Where magnetic north is now east of grid north, OS says the old rhyme no longer helps. Check the north diagram on your map before you adjust.
WHY NORTH MOVES
A restless
molten core.
BGS explains that the Earth's magnetic field changes over time because of the slow movement of molten material in the outer core. Because the field is more complicated than a simple bar magnet, compass needles line up with the local field rather than pointing straight at the magnetic north pole.
Sudden, poorly understood changes called jerks or impulses mean the predictions need refining over time. That is why BGS keeps measuring, and why OS prints the date beside the magnetic north information on its maps.
Questions
Questions on the way
What are the three norths?
True north is the direction to the geographic North Pole. Grid north runs along the grid lines on an OS map. Magnetic north is the direction a compass needle points, and it changes slowly over time.
What is the grid magnetic angle?
Ordnance Survey and BGS use the term for the angle between grid north and magnetic north. It is the angle to apply when converting between a map (grid) bearing and a compass (magnetic) bearing.
Does 'add for mag, rid for grid' still work?
Only where magnetic north is west of grid north. Ordnance Survey says that since magnetic north moved east of grid north in parts of Britain, the rhyme is no longer helpful in those areas.
Where can I find today's angle?
BGS publishes a grid magnetic angle calculator for Great Britain that gives the angle for a location. The figure printed on a map is only correct for the date it was calculated.
Do compasses point to the magnetic north pole?
Not exactly. BGS explains that the Earth's field is more complicated than a bar magnet's, so compass needles align with the local magnetic field instead.
Destination