Equipment > Compasses

Magnetic Declination - what it is and how to correct for it

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Lyle Brotherton:
We (I also used to make this mistake) tend to use, as mountain navigators, the term magnetic declination or magnetic variation (which is a synonym) when correcting a grid reference taken from map to a compass and vice a versa. Actually this describes the angle between true north and the horizontal trace of the local magnetic field and it is not the value we need to use in Great Britain when working with the British National Grid, as used on Ordnance Survey and Harveys Maps.

True North tends to be mainly on global mapping systems, which project large areas of the curved surface of the earth onto flat surfaces. However, the level of spatial distortion across relatively small areas of the Earth caused by this, such as the British Isles, can be significant. As a result, we and many other countries have developed their own local rectangular grid systems to reduce this degree of distortion. Consequently the north meridian lines on OS & Harvey Maps do not point to True North, they instead point to Grid North.

The only North-South grid-line that actually points to True North is the one that coincides with the longitude meridian 358° making Grid North west of True North to the left of this line and east of Grid North to the right.



So we need to adjust for the angular difference between Grid North and Magnetic North when converting between magnetic and grid bearings.

It is called GRID MAGNETIC ANGLE or GMA.

It is easy it is to forget to account for the difference between Grid North and Magnetic North (GMA) especially when the value is very low, such as Penzance, where is almost zero 0°24’ west of Grid North and so insignificant we do not need to account for it when converting between magnetic and grid bearings.

Yet if you travel over to the other side of England to Lowestoft, it is 3°46’ west of Grid North, and not correcting for this difference will add significant error into our navigation.

In places such as the USA this effect is dramatic, where the magnetic declination on Mount Rainier, Washington State, in the far west of the country is 16° E whereas in Portland, Maine, on the east coast, it is 16° W. If a compass were adjusted on Mount Rainier and then used in Portland without being adjusted, the error would be 32°!

As our GMA is currently West in the UK, we ‘Add for Mag & Rid for Grid’ and we must simultaneously take account of the annual rate of change.



This picture is from an actual team map currently in use, where in 2002 the GMA is stated as 3°45’ with an annual rate of change 10’ east. As today’s date is 2012 we need to multiply the annual rate of change by 10 (the number of years since data was published).

10 x 13’ = 2°10’ (where there are 60’ seconds in every 1° degree)

Therefore 3°45’ - 2°10’ = 1°35’ which is the actual amount we need to correct our compass by.

Always check the stated Grid Magnetic Angle on your map and correct it for time elapsed since it was published - if it is 1° or more, correct for it.

Pete McK:
Your book makes this clear Lyle, other books we have read do not. But this explanation is by far the best, I am guessing it was part of the editing your publisher carried out.

Can I ask if I can have the high-res images, and if it would be OK to use them in my classes at school?

Lyle Brotherton:
Sure thing Pete, will PM you thumbnails and you select which ones you want High-Res for  :)

adi:
Lyle a quick question.

With a declination of 1°35' what would you set on the compass 1°, 2° or bracket the two at 1.5°?

Lyle Brotherton:
Hiya Mate  :)

Since most baseplate compasses are graduated in 2 degree increments, for a GMA of 1°35' personally would add 2° for a grid reference taken from my map and applied to my compass and subtract 2° when transferring my bearing from my compass to my map.

I know this is a small adjustment, yet I would still correct for GMA, as inaccuracy is compound, when we take into account errors in taking the bearing, transferring it and walking on it ;)

Will bell you when back.

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