Magnetic Variation & Declination


Magnetic Variation & Declination


A short theoretical section on the subject of magnetic variation before the more practical applications within the next section.


Magnetic Variation & Declination 

Magnetic Variation & Declination



It’s important to understand the current difference between norths in degrees and minutes, and this section is theory rather than how you will use such understanding on the hill. Please read this section as the subject can be confusing.

As you might expect, the difference between grid and magnetic north is well understood by map makers, and it’s something that you need to understand too, to an extent. The difference is referred to as magnetic variation, being the difference between grid and magnetic north. (Go back and read the section relating to 'norths' if you need a refresher.)

However, you will sometimes hear people talk of magnetic declination, when they really mean magnetic variation in the UK. Declination is different, as it is the measure of the difference between true north and magnetic north, and as we in the UK don’t need to concern ourselves with True north, we can forget the proper definition of declination, other than sometimes people say declination when they really mean variation! It's an easy mistake to make, as declination is the correct term for many other parts of the world. The reason for this is that many maps in other countries do actually point to true north and do not have a third, or grid north, as we do in the UK. So, in some ways, variance and declination are the same, as they account for the difference in the map's north to magnetic north.

There is also a third term, called Convergence, which is the difference between True North and Grid North. Convergence is of no interest to the hill walker, and would be confusing to go into here.


Video

The following video explains declination by using a grapefruit! The narrator talks about declination, as he's based in the USA, but the principle is the same as variation, so the video is useful in that respect. When the narrator says "declination", think "variation", and when he says "true north", think "grid north".

Note: The example in the video is good until 1 minute and 19 seconds, but stop at that point as the remainder of the content is not relevant for the UK and will only serve to confuse!



Remember: The video talks about declination and true north. For the UK, substitute that for variation and grid north.


Important

The rate of movement of magnetic north is approx 8 minutes per year, or about half a degree every three or four years, and the current magnetic variation within the Mainland UK is approx 2 degrees west of grid north. But, quoting 2 degrees for the Mainland UK is quite inaccurate in itself, as the variation is different at different parts of the Mainland UK.

Variation varies greatly around the world, so please remember that the values quoted here for for the Mainland UK. Values for Northern Ireland are shown below.


UK Mainland Extremes

St Ives in Cornwall is estimated to have a magnetic variation of only 0 degrees 28 minutes West as at July 2012, with Peterhead in Aberdeenshire having a variation of 3 degrees 5 minutes West. As you can see, quoting 2 degrees for the UK is misleading!


Common Mainland UK Walking Areas

The variation for common walking areas in July 2012 is approx:


 Snowdon
 approx 1.5° west*
 Keswick  approx 2° west*
 Fort William  approx 1.5° west*
 Braemar  approx 2.3° west*






*remember that magnetic variation in the UK decreases by about half a degree every three or four years

In practical terms, you'd probably use a variation of 2° for all of the above examples, given that compasses normally only show a mark for every 2°. However, given the extremes shown above, it is clear that a uniform UK value is incorrect.

Most Ordnance Survey maps show the magnetic variation on the legend, or more commonly at the top middle of the map (on 1:50,000). The variation quoted will show a date reference point, i.e. July 2012, and the current annual change. Be aware that the rate of change of variation actually changes itself, meaning that what was quoted 10 years will not be accurate even if you do the mathematical adjustments.


Changes Over Time

An old copy of an OS sheet 97 (1:50,000) shows that magnetic variation in June 1983 was 6° 30’ (6 degrees 30 minutes) west of grid north with annual change of 10’ (10 minutes) east. On that basis, the rate of change would look something like this, based on one degree east every 6 years.

Date Variation Estimate
June 1983
6° 30’ west
June 1989
5° 30’ west
June 1995
4° 30’ west
June 2001 3° 30’ west
June 2007 2° 30’ west
June 2012 (5 yr gap) 1° 40’ west










In actual fact, the variation for Kendal in July 2012 – according to the British Geological Society – was 2° 10’ west, or about half a degree away from the 1983 estimates.

The maxim to follow is to know what the current estimate of magnetic variation is for the location you intend to visit, and in particular, be wary of quoted variation if your map is old. Use the tools below if you have to. Put this information on your route card, as it shows that you have taken account of variation.


Failing to Account for Magnetic Variation

In later sections it will be explained how to deal with magnetic variation in the real world, but at this point it's worth noting that if you don't take proper account of magnetic variation your bearing will be 3.5m out for every 100m walked, based on a broad value of 2° variation. That might not sound much, but the main time for using a bearing is when visibility is poor and it's likely that the distance to be travelled is much greater than 100m. 35m error for 1km, etc. Depending on the visibility, 35m could be a problem.

Click here for further discussion on this topic.


Calculation Tools
The British Geological Survey have a great tool to allow calculation of the magnetic variation based on co-ordinates or a postcode. Click here (do not use for Northern Ireland). Output example as follows, based on Keswick in The Lake District.


Source: British Geological Survey


Additional Tool

Here’s a handy tool to check the variation for any point within the UK – click here (do not use for Northern Ireland).

NOTE: the web page on the link is headed ‘declination’, but it is the value between magnetic north and grid north that is of interest to you (variation). Make sure you look at the correct box!


Northern Ireland

In Northern Ireland, maps use the Irish National Grid, which is similar to the British grid in that it is not aligned to either true or magnetic north, but its own 'grid' north. However, the Irish grid north is different to the grid north of the British grid. In practical terms, this just means that the magnetic variation used you have to take account of in Northern Ireland is noticeably different to mainland UK. 

The Ordnance Survey Northern Ireland 1:25,000 Activity Map of The Mournes shows magnetic variation being 6° 12' W of grid north in 2009, with a variation of 10 minutes East per year, thus in 2013 the variation is approx 5° 32', or 5.5° for the purposes of a compass. In practice, you'd round that to either 5° or 6° (probably better with 5° as that value will be more accurate for longer).

Care: do not to use the web links on this site to other sites that quote magnetic variation for specific locations, as they return values against the British National Grid.


Outside of the UK

Please bear in mind that this site is focussed on the UK. Should you travel abroad for walking, don't assume that magnetic declination (as you're outside of the UK) is the same value as variation in the UK. Don't forget that variation varies within the UK, and certainly does abroad. Declination can be up to 40° in Northern Canada. You have been warned!

Also remember that when travelling to certain parts of the world, you may need a different compass, which is balanced to that compass zone. See how compasses work - balancing.


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