21
Mar
2019
7

Magnetic north continues its march to the east

As expert map readers will know, when you’re out and about navigating with a compass, there is a difference between magnetic north (where the compass points) and grid north (the vertical blue grid lines shown on OS maps). And if you’re exploring in the west of Great Britain, there is a change to be aware of…

The difference between magnetic north and grid north is often referred to as grid magnetic angle and it not only varies from place to place, but changes with time too, and needs to be taken into account when navigating with a map and compass.

In 2014 there was a significant event in the changing direction of magnetic north relative to grid north on OS maps. For the first time in Great Britain since the 1660s, magnetic north moved from being to the west of grid north to the east. The change started in the very south west corner of Britain, currently affects the areas to the west of the line on our map, and will slowly progress across the whole country over the next 12 to 13 years.

Map of Great Britain with a line marking the areas now to the east of magnetic north for the first time since the 1660s

The line represents the approximate path of where magnetic north currently equals grid north

Five years on, we thought it was time for an update, so used the excellent British Geological Survey blog on the changing direction and their grid magnetic angle calculator to estimate the region where magnetic north is now east of grid north and this is what is shown on the map to the west of the red line. The line represents the approximate path of where magnetic north currently equals grid north. We also checked the current angle between magnetic and grid north for a selection of locations across Britain and estimated when the difference between magnetic and grid north will reach zero.

Location Angle (degrees and minutes) from GN to MN March 2019 (neg = west) Estimated date when GN to MN angle will be zero
Plymouth 0° 13′ May 2017
Cardiff -0° 20′ May 2020
Glasgow -0° 30′ Dec 2020
Birmingham -1° 02′ Jan 2024
Leeds -1° 20′ Aug 2025
Aberdeen -1° 32′ Oct 2025
Newcastle -1° 28′ Nov 2025
London -1° 41′ May 2028
Norwich -2° 29′ Oct 2032

 

Add for mag, rid for grid

We show magnetic north on all of our maps (and state the date it was calculated), but you can always visit the BGS site for the latest information. New maps in affected areas will also have a new icon we have created in the legend to show the new relationship between the three Norths (magnetic, grid and true).

And if you’re a fan of the mnemonic ADD for mag, RID for grid to help you remember how to make your bearing calculations…that will no longer be helpful in some areas of the country. In the south west, variation is now so small (0 degrees in Plymouth) that you’ll be able to effectively ignore it, but that will gradually change – so does anyone have a suggestion as to what will the new mnemonic be?

If map reading and using a compass is new to you, we have some helpful guides to get you started: https://www.ordnancesurvey.co.uk/resources/map-reading/index.html

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11 Responses

  1. Christian Nicholson

    We could swap round the ending of how I was taught the application of variation to read: Mag to grid-Add. Grid to mag-Get Rid.
    Really interesting to discover the variation will ‘creep’ across the country.
    For the majority of us and the small distances we travel, won’t the size of the GMA be so small for the next decade or so, as to render it almost insignificant or certainly unnoticeable on our compasses?
    Great bit of info, thank you. #EveryDayIsASchoolDay

    1. Hi Christian

      Yes, it’s 0 roughly all along that red line on the map at the moment and only has a small degree of change either side of that too, so will make little difference for the average user. It’s one to bear in mind though – and certainly spoils the old memory aid which many have been taught.

      Many thanks
      Gemma

  2. Stephen Ross

    Very interesting article. Yes, it is becoming evident that the variation will be negligible for areas close to the line, but it will vary the further you get away from it. And 1deg west or east will make a huge difference in terms of navigation. What would be useful is if the OS maps app and website could give you the magnetic variation, either as a header or footer when printed, or could be added as a layer. That would remove all ambiguity.

  3. Peter Jago

    There are various mnemonics to help remember whether to add or subtract degrees when converting from compass to true or vice versa. As a yachtsman, I need to compensate for both magnetic variation which is displayed on the compass rose on the chart (different around the UK coastline) and compass deviation which is specific to the boat. The mnemonic that I find most useful and works in all situations, at sea and on land is:

    ‘Error West, Compass Best. Error East, Compass Least’.

  4. Mike Grain

    Having had to deal with this in other parts of the world, the current mnemonic works fine if you treat magnetic west of grid as positive and magnetic east of grid as negative.
    It will be harder in 20 years when a generation of UK navigators who have been able to ignore a small difference have to sit up and take notice!

  5. DuvetCoverDrive

    Surely the most challenging aspect of this is to come up with a new mnemonic for ‘home screen, click settings, select grid, click enter magnetic deviation, enter variation’.

  6. Doug Hobbs

    The new nemonic, my inventing and I copyright it!

    Mag to grid… I just did, grid to mag…. It’s in the bag

  7. Brendan Hughes

    Another nautical mnemonic for decided whether to add or subtract is: ‘CompAss to True, aDd EasT’ (or ‘CADET’). ‘East’ means any easterly variation.

    Compass equates to Magnetic and True, Grid so you could convert this to have: ‘MAg to Grid, add EasT’ (MAGET).

    For Mag to Grid, If East variation is always added, then West is always subtracted.

    If converting from Grid to Mag, then its the reverse: East is always subtracted and West is added.

    So in practice:

    if you took a resection bearing of 256M and wanted to know what it was in Grid and the variation for your present map/year was shown as 2 degrees East, you would know to add 2 to get 258G. (‘Mag to Grid, Add East’).

    Or, to take a more complex example, if you took a bearing of 035 from your map (ie 035G), but wanted to convert for variation of 3 degrees West to know what magnetic bearing to follow, you would know you are going ‘Grid to Mag’, i.e. the opposite way from MAGAT, so West is added in this direction, so you would set your compass to 038M.

    MAGAT!

  8. Noel Darlow

    All we need to know is which one, grid or magnetic, is chosen as the reference zero angle. The other is then either a short distance anti-clockwise (negative angle) or a short distance clockwise (positive).

  9. John Nightingale

    I remember when it was 6 degrees. I challenge most users, including myself, to take bearings within 2 degrees, so I think you are safe to just ignore variation in the UK for the nexrfew years.

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