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.
If you can come up with a better mnemonic that is applicable for all circumstances Susan Macmillan would like to hear from you!
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.
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
Is that true?
thank you for your time
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.