A compass needle seldom points directly to the north pole, because Earth’s magnetic fields pull the compass needle towards what is known as magnetic north. Because the angle between true north, the direction from you towards the north pole, and magnetic north varies from place to place, we must account for that variation when navigating. This difference is known as declination. It’s different from Magnetic Deviation, which is a local magnetic field creating an error.
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The terms variation and declination refer to the same feature. On a map refer to it as declination. On a chart refer to it as variation.
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Magnetic declination, also called variation, is the difference between true north and magnetic north. It is either east or west. East declination is considered positive and west is negative. Charts or maps define the area’s declination usually on the compass rose. In the above pictured chart, you can see that the inner compass rose shows an 11° west variation. Declination varies over location and time. For example, in Chesapeake Bay the declination is 11° west. Across the U.S. in Wainwright Inlet, Alaska, the declination was 19°30′ east in 2003. In 2011, it was 21°38′ east. In 2011, the difference between Chesapeake Bay on the U.S. east coast and Wainwright Inlet was 32°. If you somehow calibrated your compass for paddling in Chesapeake Bay and didn’t change that in Alaska, you could end up far off course. We can calculate how far by running the distance paddled and the degrees off through a formula. (This formula is useful for doing the navigation technique known as Aiming Off.)
Distance from destination = tan(degrees off) * distance to destination
Let say we’re paddling 1.5 miles. At a 32° error, we’d end up almost a full mile away from our destination. A simple way to calculate the error in feet is to take the distance paddled multiplied by the degree error multiplied by 92.
Distance from destination = distance to destination * degrees off * 92
Instead of worrying about errors, we correct for the declination while navigating. The process involves adding or subtracting the declination from true north or your compass reading to arrive at the right measurement. The only time you need to make the correction is when you take a course bearing from the map, which is aligned to true north, to your compass, which aligns to magnetic north. Or when you want to take a compass bearing back to your map. When going from map to compass, you add the degree of west declination and subtract the degree of east declination. When going from compass to map, you subtract the degree of west declination and add the degree of east declination. It gets confusing, but a simple rhyme helps in memorization.
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When going from map to compass;
west is best and east is least.
When going from compass to map;
stay found by turning the rhyme around.
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The rhyme reminds you to add for west declination, subtract for east declination and to turn the rhyme around when going from compass to map.
For example, in Chesapeake Bay the declination is 11° west. If you take a course bearing of 45° from your chart, then you add (west is best) 11° to that when taking it to your compass. Your magnetic course bearing — where your compass points — is 56°. If you take a bearing to a distant buoy and get 90°, when taking that bearing to the map, you subtract (opposite of west is best) 11° from 90° to get a true bearing of 79°. If you had the same bearings in Wainwright Inlet, Alaska with a declination of 21° east, the you’d subtract 21 from 45 when going from chart to compass (east is least) and get 24°. You’d add (opposite of east is least) 21° to your 90° bearing to the buoy to get 111° to take that to the chart.
Marking Declination or Variation on a Map
When marking course bearings on your map or chart, note whether the degrees are in magnetic or true. I like to use both, the true bearing first followed by the magnetic bearing. See an example in the top image in the Course, Bearings and Headings article.
You can avoid doing math by drawing declination lines on your map or chart. When taking a course bearing or transferring a bearing to the map or chart, you align your compass to the declination lines instead of the meridians. To draw declination lines on your map run a line through the compass rose at the angle of the map’s declination. Then draw lines parallel and 1 inch apart across the rest of the map. Penobscot Paddles has a good article on the process: Magnetic North Lines – The Path to Happiness
On some maps, like Canadian topo maps, the lines on the map are grid lines instead of latitude and longitude lines. Grid lines might vary from true north. To account from the difference between magnetic north and grid north, you use grid declination instead of true declination when doing your addition or subtraction. For example, in the image magnetic north is east of grid north but west of true north on this Canadian topo map.
If you read my Top Paddling Gear of 2010 article, you know I’m a fan of simple compasses. My favorite compass is the simple Silva Starter 1-2-3 Compass. Not everyone likes simple compasses. Just a step-up from the Sivla Starter is Suunto’s M-2D Compass. The Suunto is double the price of the Starter, but it includes an adjustable declination scale, which means you can set the scale and forget about doing math, unless you use a kayak mounted compass too. Then you’re stuck doing the math. For canoeists, the Suunto makes good sense. No more confusion or figuring out what west is best means.
Magnetic Declination Calculator
The Geological Study of Canada has a free declination calculator on its website. You enter your location’s latitude, longitude and year, then the calculator provides the declination for that time and place.