North by Northeast: The Trouble with the Earth’s Shifting Magnetic Field

A photograph taken at Loch Lomond. Image: NaturPhilosophieA Rambling Geomagnetic Pole

Magnetic North made an unusual and historic shift.  For the first time in more than 220 years of map making, Ordnance Survey has noted that North lies East, and not West, of Grid north for parts of Southern Britain.  But how does this shift in magnetic field affect map reading in Scotland’s hills? 

Scientists have known for a long time that the Earth’s magnetic field is fluctuating, even fading…  What is more uncertain is whether the weakened field is on its way to complete a collapse and reversal that would essentially “flip” the North and South magnetic poles, causing compasses pointing North to point South instead…

The geomagnetic field is the magnetic sphere that extends from the depths of the Earth’s interior to the point where it meets the solar wind – a stream of solar particles, emanating from the Sun’s violent activity.  Above the ionosphere, the magnetosphere region extends several kilometres into space.  The field magnitude at the Earth’s surface ranges from 25 to 65 micro-Tesla.  The Earth’s field is comparable to that of a magnetic dipole tilted at a 10° angle with respect to the rotational axis, as if a bar magnet was placed at the centre.

Unlike a bar magnet, though, the Earth’s magnetic field changes over time.  Both geomagnetic poles move over time because the magnetic field of Earth is produced by the motion of molten iron alloys within the outer core.  The dipole reversal pattern is erratic, and it is hard to pinpoint when it would happen…  But it has happened in the past…

A dipole reversal is a rare event.  The last time one occurred was 780,000 years ago.  The time between dipole reversals varies from a thousand years to millions of years.  Seeing how the Earth’s magnetic field has fluctuated over the geological time scale is even possible.

 

The Geological “Tape” Recorder

A diagram showing how oceanic stripe form magnetic anomalies in the Earth's crust.The seafloor acts as a kind of geological ‘tape recorder’.  By studying seafloor sediments and lava flows, scientists can reconstruct the magnetic field patterns of past ages.  As magma seeps out of the mantle and the lava cools and solidifies to become new ocean floor, the magnetic minerals in it are magnetised in the direction of the magnetic field.  Thus, starting at the most recently formed ocean floor, one can read out the direction of the existing magnetic field in previous times as one moves further away to older ocean floor.

Over the past 150 years, the poles have moved westward at a rate of 0.05° to 0.1° per year, with little net North or South motion.

Over the last century and a half, since monitoring began, scientists have measured a 10% decline in the dipole.

 

Current Location for the Magnetic ‘North’

The north magnetic pole moves over time due to magnetic changes in the Earth’s core.

The Geological Survey of Canada has determined its exact location over the years:

So, the North Magnetic Pole wanders, but it occurs slowly enough that an ordinary magnetic compass does remain useful for navigation.

 

How does it actually affect hill-walkers in Scotland?

Those who navigate the Great British landscape by map and compass will know there are three Norths.

Grid North, True North, Magnetic North

  • Grid North is the direction of a grid line, which runs parallel to the central meridian of the National Grid – the reference system on Ordnance Survey maps.
  • True North is the direction of a meridian of longitude, one of the imaginary circles of the Earth, which all converges on the North Pole.
  • Magnetic North is the direction magnetic compasses point to.  Magnetic north is always shifting, ever so slowly, under the influence of changes within the Earth’s magnetic field, itself affected by changes to the spinning of the planet’s core.

Hillwalkers may well have broken into a cold sweat when reading recent attention-grabbing headlines about magnetic north moving east.  Not having this up-to-date knowledge may conjure up visions of walking innocently over a cornice and tumbling to your doom in white-out conditions.

But it is not quite as dramatic as that.

A close-up photograph showing a map and a compass.How to use a Compass and Map

Place your compass on the map.  Make sure the ‘direction of travel arrow’ is pointing in the direction of your chosen route across the map.  The easiest way to line the arrow is to place the side of the base plate, so that it crosses your current location point and the next destination on your journey.

Holding the compass base plate still, you need to turn the compass housing in order for the orientating lines to match up with the eastings (the vertical, north–south lines) on your map.  Holding the map flat and the compass still, you then need to rotate your body so that the compass needle settles in line (opposite) with the index line.  To fully orientate your map, you must make adjustments for the magnetic variation.

 You can properly orientate the map by carefully turning the compass housing 4º clockwise, and then turning your body again to realign the magnetic needle with the index line.  Your map is now oriented to the North.

Find out more about map reading skills with the British Ordnance Survey‘s Map Reading Made-Easy Leaflet.

 

Current Magnetic Variation

Maps are aligned to ‘Grid’ North.  Compasses indicate the direction of Magnetic North.  There is a difference between the two, which is the result of trying to reproduce a spherical world on a flat map.  This difference varies with time and depending on where you are in the World.  For example, the difference between grid and magnetic north in Scotland is very different to that in South America.

For the geeks, British Geological Survey has a website where you can enter your location details and receive the current magnetic variation for your chosen location.

If the result appears to be west of Grid North, the user needs to “add on” to the bearing that has been taken from the map.  For example, a bearing from the map in the Cairngorm area was 48 degrees.  The user should add on a further two degrees and walk on a 50 degree bearing.

If the result bears east of Grid North – as is now the case in deepest Cornwall – then this figure should be deducted.

However…

Each tiny black line on the compass housing represents two degrees, so it is impossible to differentiate the tiny adjustments required for 100% accuracy.  When factoring in the human element of walking along the bearing, this degree of accuracy is probably irrelevant.  Although in winter white-out conditions, near corniced edges, the safest thing to do is probably adding those two degrees.

Over the next few years, here in Scotland, this variation ought to decrease gradually to zero.  One less thing for Scottish mountaineers to worry about…  For a time…