Early November 2022 will see geospatial history being made when true north, magnetic north and grid north combine at a single point in Great Britain for the first time ever.
According to calculations made by Ordnance Survey (OS), Great Britain’s national mapping service, the historic triple alignment will make landfall at the little village of Langton Matravers just west of Swanage in early November and will stay converged on Great Britain for three and a half years as it slowly travels up the country.
The three ‘norths’
When you’re 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).
Grid north is the blue lines on an OS map that either points directly to, or near to the North Pole, whereas true north is the direction of the lines of longitude that all converge at the North Pole.
Across OS maps true north varies from grid north since it reflects the curve of the earth, except on one grid north line, which aligns with longitude 2 degrees west of the zero Greenwich meridian line.
Anywhere on this ‘special line’ grid north and true north align.
Magnetic north marks the northward line to the magnetic North Pole.
The position of the magnetic North Pole and the direction of magnetic north moves continually due to natural changes in the Earth’s magnetic field.
After always being to the west of grid north in Great Britain the last few years have seen magnetic north move to the other side of grid north.
The change started in 2014 at the very tip of Cornwall and is slowly moving west to east across the country.
It is now reaching the ‘special line’ and will incredibly converge with the other two ‘norths’ for the first time in history.
After making landfall at Langton Matravers, the triple alignment will pass northwards through Poole by Christmas and then Chippenham and Birmingham before reaching Hebden Bridge in Yorkshire in August 2024.
It will pass though the Pennines before leaving the English coast at Berwick-Upon-Tweed a year later in August 2025 and it does not hit land again until around May 2026 at Drums, just south of Newburgh in Scotland. After passing through Mintlaw its last stop in Scotland and the landmass of Great Britain is Fraserburgh, around July 2026.
Due to a number of factors these predictions are likely to change, but only by a few months.
Factors include updated magnetic field observations from the British Geological Survey (BGS) due to changes in the flow of iron in the liquid outer core.
Mark Greaves, Earth Measurement Expert at Ordnance Survey, said: “It is no exaggeration to say that this is a one-off event that has never happened before.
“Magnetic north moves slowly so it is likely going to be several hundred years before this alignment comes around again.
“This triple alignment is an interesting quirk of our national mapping and the natural geophysical processes that drive the changing magnetic field.
“But for navigators the same rules will apply whether they are simply on a trek or a walk or flying planes or navigating ships at the other end of the spectrum.
“They will always have to take account of the variation between magnetic north from a compass and grid (or true) north on a map.”
As part of its long-running collaboration with OS, the BGS geomagnetism team makes detailed measurements of the magnetic field at 40 sites around the UK.
These enable scientists to create high-resolution maps and to make accurate forecasts of the changing declination angle.
Dr Susan Macmillan, of the BGS said: “This is a once in a lifetime occurrence.
“Due to the unpredictability of the magnetic field on long timescales it’s not possible to say when the alignment of the three norths will happen again.”