Friday, 31 January 2014

Something’s happening to Magnetic North in Great Britain in 2014 by Susan Macmillan




The magnetic field of the Earth is changing slowly every day. This year, for the first time in 350 years in Great Britain, we’ll see the direction of magnetic north move from being west of grid north to east of grid north. Susan Macmillan of the Geomagnetism Team explains what’s happening and what’s in store for compass users in Great Britain over the next few years.



Estimates (Jan 2014) of grid magnetic
angle at mid-2014
and its annual decrease.
Magnetic north is west of grid
north by
the amount shown. Red-shaded region is

where it is EAST.
At the BGS we derive a model of the Earth’s magnetic field valid for the area of Great Britain using data collected at three magnetic observatories and a network of repeat stations. A new model is derived every year to keep accurate track of the slow changes in the Earth’s magnetic field. This model is used to calculate the angular difference between the directions of grid north and magnetic north, otherwise known as grid magnetic angle (GMA). The map shows how grid magnetic angle currently varies across the country, and also how it varies in time. 


In the bottom left corner you can see where magnetic north is east of grid north. It will take approximately 20 years for the rest of the country to see magnetic north change from being west to east of grid north. The last time magnetic north was easterly in the UK was over 350 years ago in about 1660 when it was recorded by more than one observer in and around London. Since then we have had varying grid magnetic angle with the maximum being about 27° west in Shetland in 1818. 


You can calculate grid magnetic angle by going to the BGS Geomagnetism website and using the GMA calculator. The calculator outputs a grid magnetic angle for any given location (entered as a British National Grid reference, latitude and longitude or postcode) which you can then use with a magnetic compass and map. To learn about using a compass and map read this blog by the Ordnance Survey.



Grid magnetic angle and its estimated annual rate of change are shown on Ordnance Survey maps.  Because of the changes in 2014, the OS have had to design a new icon to show the new relationship of magnetic north compared to grid north.  


What is causing this gradual change in direction of magnetic north in the UK?
 

Now the difficult stuff. The Earth’s magnetic field is sustained by a dynamo process in the liquid outer core of the Earth. Interactions between the flow of the molten iron-rich material in this region and the magnetic field generate electrical current that, in turn, creates new magnetic energy which sustains the field. Energy sources for the fluid motions are primarily convection - as the Earth slowly cools down, warmer fluid rises and cooler fluid falls and solidifies onto the inner core. This in turn changes the chemical composition of the fluid, and buoyancy forces result. The rotation of the planet also contributes. This dynamo process also results in the movement of the magnetic north pole but because the Earth’s magnetic field is more complicated than dipolar magnetic compasses do not point directly to the magnetic north pole. Compass needles instead align themselves with the local magnetic field.


What does this mean for compass users in Great Britain?


This change will affect ramblers and hill-walkers who use grid magnetic angle to correct between magnetic bearings and grid bearings. A common mnemonic to help remember whether to add or subtract the correction, “grid to mag, add – mag to grid, get rid”, will unfortunately become redundant when magnetic north becomes east of grid north. Up till now a westerly grid magnetic angle is added to a grid bearing to convert it to a magnetic bearing, but from 2014 and onwards whenever you see an easterly grid magnetic angle in the margin of the map you need to subtract the angle from the grid bearing.

A mnemonic that will work after the change is “East is least, west is best”. This mnemonic is applicable anywhere in the world, no matter whether magnetic north is west or east of grid north. The other nice thing about this mnemonic is that it is also applicable with any type of map with north lines. The north lines can be either grid north lines or true north lines as on mariners’ charts. “Least” in this context means “subtract” and “best” means “add”.


However this mnemonic only works if you are converting from map to magnetic bearings. This is the most common use but if you are applying it when locating yourself on a map by two intersecting magnetic bearings to nearby identifiable features, you have to remember this because in this case you are converting from magnetic to map bearings. 


Susan Macmillan

If you can come up with a better mnemonic that is applicable for all circumstances Susan Macmillan would like to hear from you!






12 comments:

Alastair Lings said...

When I was a navy cadet I was taught CADET, Compass>>>True ADd East. We had to take in to account the magnetic declination, and the deviation due to the ships magnetism which varied with direction.

Trollockillsrand said...

How will this affect my Feng Shui? Will I need to reposition my bed?

Gardendata said...

Mmmmm. Not sure how I got to this page - other than Google which normally gets me to where I want to go. It failed in this instance, so maybe it is not aware of your findings and observations!!! (But I rather enjoyed getting lost this once!,,) :-)

Ron said...

Fascinating. I bet the Mountain Rescue Teams will be busy for a while, until the old die-hard map and compass people adjust.

dyvroeth said...

MR Lings is spot on. "Compass to True, Add East" - from my GCE O Level Navigation syllabus in 1974. Will OS and the Hydrographic Office be running a major set of reprints of maps and charts as a precaution ? What about a special NOTAM and NOTMAR to highlight the event ?

BGS Geomagnetism said...

Thanks for your comments. CADET is a popular one but is strictly speaking only applicable for maps and charts where the north lines are true north lines and not grid north lines. And as one is normally going from "True" to "Compass", you have to remember to apply it in reverse, i.e. subtract easterly magnetic variation/declination. Perhaps additional information that is easy to forget in the heat of the moment. The lack of specificity of the type of north in "East is least, west is best" means that it is applicable for both types of maps.

I very much hope the Mountain Rescue Teams will not be busy on this account. Grid magnetic angle is quite small as we go through this transition.

Susan

Costis Panaretos said...

When the time arrives to have Easterly Declination, another mnemonic could be:
Map to Field = Get Rid
Field to Map = should Add

when are we expecting these changes? (mid 2014 means?) are they gonna cover the whole UK? when these changes are here, are they gonna stay for 350 years?

thank you for your time

BGS Geomagnetism said...

Declination is the angle between true north and magnetic north. Grid magnetic angle is the angle between grid north and magnetic north. In the UK we are not expecting to see easterly declination till about 2018. However, because of the relatively large convergence angles in the UK (the angle between true north and grid north), the arrival of easterly grid magnetic angle precedes that of easterly declination. It starts in 2014 in the SW and will take about 20 years for the whole country to be affected. Easterly grid magnetic angle is likely to be around in the UK for more than a century, but it's hard to say exactly how long.

Costis Panas said...
This comment has been removed by the author.
Costis Panas said...

So, actually for the hill walkers of the country what counts is if you have easterly or westerly grid magnetic angle and not easterly or westerly declination (east or west from true north, where true north coincides with the line of longitude 2oW). Meaning that if the magnetic north is west of grid north but still east of true north, we need to change the old mnemonics in order to navigate with map and compass.

Is that true?

thank you for your time

BGS Geomagnetism said...

Hill-walkers are using maps with grid north lines so yes, what matters is whether grid magnetic angle, rather than declination, is easterly or westerly. Declination is important for mariners using sea charts, where the north lines are true north lines i.e. lines of equal longitude.

For topographic mapping in Great Britain using the transverse Mercator projection (the basis of the British National Grid), along 2 degree W longitude, true north and grid north are in the same direction. West of this, true north is east of grid north and east of this, true north is to the west of grid north. There's a useful, though rather detailed, document available from the Ordnance Survey at http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf describing the coordinate systems in Great Britain and I find Figure 8 handy for visualizing the relationship between grid north and true north. The angle between grid north and true north is called grid convergence and, if you're really keen, can be computed using a spreadsheet available at http://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/coordinate-spreadsheet.html.

Till now magnetic north has been west of both for the whole country but over the next few years will slowly become east of grid north and west of true north for the area west of 2 degrees W. After about 2018 and for the area east of 2 degrees W, magnetic north will slowly become east of true north but remain west of grid north for a while, and eventually for the whole country, will be east of both. But for hill-walkers using compasses it is the relationship between grid north and magnetic north that is important. True north can generally be ignored, unless doing very accurate night navigation using the pole star!

Thank you for your interest.

vasudha dharani said...
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