Magnetic North, True North, and the Importance of Declination

I recently changed the declination setting on my compass from 11° 22.26′ West to 22° 49.56′ West when I moved from Peterborough, Ontario to Nain, Nunatsiavut, Labrador. If you have ever used a compass to follow directions or navigate to a point on a map and you are unsure what I’m referring to here, you may have found yourself slightly off your target.

Though many of us likely still rely on topographic maps of some sort, whether these are on paper or on an electronic GPS, I realise that most people probably don’t navigate from one point to another using an azimuth and compass anymore. However, if you are one of those people (and I am) who enjoys the idea of using an old-timey, romantic method of navigation presided over by the Earth’s ancient magnetic field and the artistries of cartography, it is important to understand the concept of declination and how to calibrate your compass correctly.

This is an admittedly somewhat simplified explanation of the topic of magnetic declination. If you would like to really understand the intricacies of the Earth’s magnetic field, I encourage you to do some deeper research. However, for most of us who just want to be sure we get from Point A to Point B, the basics as presented here will suffice.

Magnetic North vs. True North

If you have ever used any kind of official map – including topographic maps while hunting or on backcountry trips – you might have noticed what looks like a rather complicated set of North arrows. The map will also have vertical and horizontal grid lines. Understanding the relationship between the various North arrows and being able to make sense of how to use your compass with these arrows is critical to getting where you want to go.

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There are three components to a map’s compass diagram. First, the Magnetic North arrow is aligned with the Earth’s magnetic field. Second, the True North (often abbreviated to “T.N.”) arrow is aligned with the Earth’s rotational axis and geographic north. Third, Grid North is aligned with the grid lines on the map, which were determined based on the aerial photograph used to create the map.

The Earth’s magnetic field is created by a dipole magnet: imagine a straight bar magnet running right through the Earth with a north and south pole. The axis created by this dipole magnet is offset from the axis on which the Earth rotates by roughly 11 degrees.

Magnetic and geographical pole of the Earth
The difference between the magnetic and geographic poles. Credit: Mila Gligoric – Fotolia.com

The Earth’s magnetic north pole is located near Ellesmere Island in the Canadian Arctic. The Earth’s geographic north pole is the point where the lines of longitude converge. Because of the irregularity of the Earth’s magnetic field, a compass doesn’t necessarily point directly to the magnetic north pole; however, for all intents and purposes, think of your compass as navigating you in the direction of magnetic north (for a more complete description of this, click here).

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The Earth’s magnetic field is irregular and highly elliptical. The red star is the location of magnetic north. Source: NRCAN

What is Declination? 

When we use a map and compass to navigate, we are actually using true north to determine our direction of travel; however, if we just follow our compass needle, it will eventually bring us to magnetic north. We need to calibrate our compass so that it compensates for the angle between magnetic and geographic north, referred to as declination. This allows us to use the compass needle, which points at magnetic north, to navigate along a path referenced to true north.

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The Earth’s magnetic field and declination values for different locations. Source: NOAA

Determining and Setting Declination on a Compass

When purchasing a compass, I suggest you choose one that has an adjustable declination. It is possible to make the mental calculation and manually adjust your compass each time you use it to navigate, but being able to set the declination and have the compass do the work for you is much better. I have had a lot of good success with Suunto products. For a compass, I have been using a Suunto MCA-D for the past seven years and it has worked very well. I also use a Suunto Vector watch, which has a compass and a declination setting.

To set the declination on your compass, you need to know the angle between magnetic north and true north and whether declination at your location is East (positive) or West (negative). The key point is that declination is not the same for every place on the planet.

The compass diagram on a map will tell you the declination for the location the map covers; however, the location of magnetic north changes each year and it changes at a different rate depending on your location. This means that declination values will also change, meaning we can’t just find a static value for declination, set the compass once and be done with it forever. This information is supplied on the map. Look again at the example from Peterborough, Ontario:

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With this information, you can calculate the declination in the year the map was made, the amount the declination has changed since that year, and the declination in the current year. If you are interested in learning to do these calculations yourself, Natural Resources Canada (NRCAN) provides a good breakdown of the process. These days, however, there are some good websites that will show you the current declination in your location, and due to the variations in the magnetic field, these sites are actually more accurate than manually calculating the value based on maps. Again, NRCAN provides one based on your coordinates and I also recommend this map-based sites that allow you to simply find your location on a map.

Once you have the declination value for your location, you can set it on your compass. Again, it is critical to correctly set the declination as East (positive) or West (negative). The rule is that if you are east of true north, your declination setting will be West and vice versa.

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The declination on this compass is set on the underside of the housing. This compass is set to a declination of 22 degrees West.

Navigating With a Compass

The actual effect that setting declination has on your compass is to offset the North arrow indicators by the number of degrees of declination. It can be somewhat confusing because if you want to travel due north, the needle on your compass will actually be pointing slightly off from the direction you are walking – that is your compass compensating for declination. If you have not set the declination on your compass, you can imagine how the farther you walk following an azimuth will bring you further off track from where you intended to end up.

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This compass now has needle match: the floating needle is lined up with the North arrow indicators, which you can see are offset 22 degrees from the “N” at the top of the housing.

Happy Compassing

Using a compass can be a lot of fun. I carry a GPS with me when I’m out in the field, but I also always have a compass that I still like to mess around with just to make sure I keep those old school skills strong. Learning how to navigate with the Earth’s magnetic field allowed humans to move around the globe with a much greater understanding about where we would eventually end up.

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Edmund Halley’s 1700 declination chart.

The first known measurement of declination is credited to the Chinese around the year 720. China is thought to have been widely using compasses since the 11th century. At least since the 13th century, European explorers have been perplexed by the shifting nature of magnetic north indicated by their compasses, and it was by coming to understand the concept of declination that this phenomenon became clear.

When most people hear the name Edmund Halley, they probably think of the comet that visits our skies every 75 years or so. Edmund Halley was an English astronomer who worked in the Enlightenment era of the 17th century. In addition to his work on what later became known as Halley’s Comet, he also produced what is thought to be the world’s first declination chart in 1700. Halley plotted declination as contour lines on a map, which is now a standard method to chart declination. Learning how to navigate using Earth’s magnetic field is a rich part of outdoor skills and adventure.

Do yourself a favour and buy a compass and a map. Head out, pick a point, and see if you can navigate yourself to that point. Effective compassing is a skill and an art that takes practice, and trust me, it’s more difficult than simply burying your head into a compass and walking in a straight line through the woods. With the conveniences of today’s handheld GPS devices, learning to use a compass can bring you back in time and be a lot of fun.

 

 

 

 

 

 

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