Unfortunately, your compass doesn’t work straight out of the box. The north on you map isn’t quite the same as north on the compass, because of this we need to adjust for magnetic declination
Magnetic declination is the difference between the geographical north pole and the magnetic north pole according to your compass, it occurs because there is not just one north but actually three. Adjusting for magnetic declination is essential for accurate use of a compass, especially if you are traveling any distance, as over even relatively short distances you can get quite off track from where you would expect to be heading. Because of this, it is essential that you understand how to calculate and adjust for magnetic declination.
The Three Poles
True North is the in the same line as the meridian of longitude which all converge on the North pole. This is where our maps point to as north and is the axis in which the earth spins and is called the Geographical North pole which True north will take you to.
Magnetic North is where your compass points to. The magnetic north pole is constantly changing location and may one day even flip so that the poles are the other way round. This is the reason we need to adjust for the declination as we need to account for the fact that north on our map is not north on the compass to avoid errors in navigation.
Grid North is what you find on the National grid which is our grid system in the UK. The grid lines on your map can vary from true north as much as 4° in some places of the country, therefore, it is important to take this into account. In the bottom of an OS map is a grey shaded box which will say what you have to account for to make up for the difference between the grid north and magnetic north.
How to find out the magnetic declination for my area
To find out the declination for where you are is possible in a few different ways. The first thing we need to determine is whether which direction we need to adjust for declination.
If True North is East of Magnetic North then this is called Easterly Declination.
If True North is West of Magnetic North then this is called Westerly Declination.
Next, is to find the angle of difference.
On your Map
Usually at the bottom with the legend, your map should have the information you need to work out the magnetic declination for where you are. On an OS map it may say something like this:
At the centre of the E and W sheets true north is 1°27′ and 1°42′ east of grid north respecivly. Magnetic north is estimated at 1°57′ and 1°49′ west of grid north respectivly in july 2010. Anual change is roughly 09′ east.
From this we can work out our angle of declination from the grid north to the magnetic north. If we were using the East sheet then in 2010 the declination would be 1°57′ West.
Then we need to account for the annual rate of change which is aproximatly 09′ east. Note that the rate of change does change aswell so if you have a really old map then it may be an idea to check the rate of change against another method below.
So in the year 2016, we are 6 years on from 201o, 6 x 09′ is 54′. Subtract 54′ from 1°57(because the declination is east but change is to the west) then we get 1°03′.
1°03′ east is our magnetic declination so we would adjust out compass by 1 degree.
Online lookup
Another way you can find out is online. These resources will give you an accurate and quick angle for you to work from.
http://www.magnetic-declination.com/
http://www.ngdc.noaa.gov/geomag-web/#declination
http://geomag.nrcan.gc.ca/calc/mdcal-en.php
Or some satnavs and GPS devices may give you the information you need.
How to adjust for magnetic declination on the ground
Now that we have worked out our magnetic declination we now need to adjust our bearings to reflect the difference.
With your Compass
The easiest way to do this is with a ‘set it and forget it’ compass. Some compasses come with a screw to adjust for declination on the compass housing meaning that you can set the compass to account for the declination and then use the compass as normal without having to think or change your bearings.
If you have one of those compasses then on the bottom you will find a small screw. To do this:
- Find your magnetic declination.
- Turn the compass housing so that north is inline with the index.
- Find a small brass screw on the bottom and use this to adjust your compass.
- To adjust East, turn the screw Clockwise.
- To adjust West, turn the screw Anti-clockwise.
- Turn the screw the required amount to set the magnetic declination for your local area.
This technique reduces your chance of error when navigating in a specific area and makes it a lot easier out on the field.
Remember: if you change location you will need to re adjust the compass for the local area.
On the ground
If you do not have a compass like that or just want to practice for when you have a standard compass then we can also do the adjustments on the ground each time you are taking a bearing. The procedure is opposite depending on whether you have East or West declination so take note of and remember at least one of these rules, then for the opposite you just do the opposite.
If the declination is West
If you are taking a bearing from the map to use on the ground then you need to Add the declination to your compass.
If you are taking a bearing from a feature on the ground to use on the map then you need to Subtract the appropriate declination for the ground.
If the declination is East
If you are taking a bearing from the map to use on the ground then you need to Subtract the declination to your compass.
If you are taking a bearing from a feature on the ground to use on the map then you need to Add the appropriate declination for the ground.
Summary
So that is it, magnetic declination. Always try and find out the declination for your area before you go out onto the hills, write it down on your map or route card so that it is always to hand and then remember to adjust for it on each leg if you do not have an adjustable compass.
It is important that you understand these techniques as most places in the world will require you to adjust for this, sometimes as much as 25° or more so if it doesn’t make sense just yet then give it a read through again till you get it.
As always, if you have any questions then please ask us in the comments and we will get back to you with an answer.
