What Do You Mean, Three Norths?

The Ordnance Survey is making a small deal over a so-called “triple alignment” of true north, magnetic north and grid north early this month: “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.”

Now, grid north is an artifact of a map projection’s grid lines. On a map grid there’s always some difference between true north and grid north except along the central meridian, which in Ordnance Survey maps is two degrees west of Greenwich. The further away from that central meridian, the greater the difference.

What the Ordnance Survey is hyping is that magnetic north, which is constantly shifting, has moved to a point where magnetic declination (the difference between true north and magnetic north) is zero along that central meridian. Kind of neat—if you’re using an Ordnance Survey map. Because this particular triple alignment only exists for Ordnance Survey maps. It’s all a bit anglocentric, really (especially the bit in the video that describes true north as “the line which runs through Britain to the North Pole”).