magnetic declination – The Map Room

What Do You Mean, Three Norths?

Jonathan Crowe — Thu, 03 Nov 2022 15:40:35 +0000

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”).

World Magnetic Model Being Updated a Year Early

Jonathan Crowe — Thu, 10 Jan 2019 18:02:26 +0000

The World Magnetic Model—the standard model of the Earth’s magnetic field and a crucial part of modern navigation systems—was last updated in 2015. That update was supposed to last until 2020, but problems with the model started within a year of the last update. As Nature reports, a geomagnetic pulse under South America in 2016 made the magnetic field “lurch”:

By early 2018, the World Magnetic Model was in trouble. Researchers from NOAA and the British Geological Survey in Edinburgh had been doing their annual check of how well the model was capturing all the variations in Earth’s magnetic field. They realized that it was so inaccurate that it was about to exceed the acceptable limit for navigational errors.

As a result, the WMM is being updated a year early—this month, in fact, though the U.S. government shutdown is pushing back the release of the updated model.

Birds Discover Longitude

Jonathan Crowe — Tue, 21 Nov 2017 17:00:58 +0000

Eurasian reed warblers don’t need no stinkin’ marine chronometers. A new study suggests that the migratory birds make use of magnetic declination to determine longitude, “at least under some circumstances under clear skies. Experienced migrants tested during autumn migration in Rybachy, Russia, were exposed to an 8.5° change in declination while all other cues remained unchanged. This corresponds to a virtual magnetic displacement to Scotland if and only if magnetic declination is a part of their map. The adult migrants responded by changing their heading by 151° from WSW to ESE, consistent with compensation for the virtual magnetic displacement.”

Not, it would seem, accurate enough for the species to earn a chunk of the Longitude Prize, and it’s not like John Harrison should have been messing about with birds instead of clocks, but interesting all the same. [GeoLounge]