How to Navigate the Backcountry

Magnetic Declination
Gorp.com
Lines of Declination Illustration
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Magnetic declination, also known as variation and deviation, is the difference in degrees between true (polar) north and where the earth's magnetic lines of force are actually focused. Declination fluctuates to a minor degree, since the magnetic lines of force themselves fluctuate in intensity. These magnetic lines of force currently come together in the Queen Elizabeth Islands region in northern Canada. Your compass's north-seeking arrow pivots within the housing to point at magnetic north, so to account for this deviation between magnetic and true north, you must either add or subtract the difference in degrees between the two norths, unless you happen to be standing on a line of no variation/deviation, called the agonic line. You can find that difference within the margin (you do have your topographic map with you?) in what is known as the declination diagram, shown here.

Note the difference marked in degrees between true north and magnetic north. In order to account for this difference so that your azimuths are accurate, you must orient your map and compass together and remove the difference. It's simple.

Start by spreading out your map on some level ground in front of you with the bottom of the map closest to you. Now set your compass's straight edge along the true north line in the declination diagram with the compass's bezel ring set on directly north. Turn the map and compass together until the arrow (needle) and map are both facing directly north. If you have westerly declination, you must remove the difference between true and magnetic north by turning the map and compass together to the east (right) exactly the number of degrees of declination shown in the diagram, making sure the compass remains aligned on the true north line of the diagram. If you have easterly declination, you will turn everything to the left, or west.

Watch your compass needle closely and stop when it is pointing at the bearing in degrees that is shown in the diagram to the left or west of the 'N' on your bezel ring. So, if you were standing atop Saddleback Junior near Rangeley, Maine, where your declination is eighteen degrees westerly, you would turn your map and compass to the east (right) eighteen degrees. At this point your needle should be pointing at 342 degrees, which is eighteen degrees less than 360 degrees, and is to the west of north. Now, if you had eighteen degrees easterly declination instead of westerly, you would turn everything to the left or west, and your needle would be pointing at eighteen degrees, which is east of north.

At this point your map and compass are aligned, and all bearings taken from this point are true, since you have removed the difference between the two norths. There is no need whatsoever to add or subtract degrees when you shoot a bearing now.

Another method of getting your map and compass in sync is to turn your bezel ring the number of degrees of declination for that area. Lay the compass's straight edge along the true north line again and orient your compass and map to magnetic north. If you have eighteen degrees westerly declination, shown on your declination diagram with the magnetic north line veering off to the left of true north, you must now turn your bezel ring eighteen degrees to the east. If you have eighteen degrees easterly declination, shown on your declination diagram with the magnetic north line veering off to the right of true north, you must turn your bezel ring eighteen degrees to the west (342 degrees). Now turn both your map and compass until the needle centers on the alignment arrow on the base of your compass. You're oriented. A simple way to remember this procedure is RALS, for Right Add Left Subtract. If the magnetic north line on the declination diagram is to the right of the true north line, you add the declination to 360 degrees. If it is to the left, you subtract.

Now that your map and compass are oriented, you have to figure out where you are. That is, since you are lost, you have to figure out where on the map you are, so that you can get to somewhere else on the map where you will not be lost. This can be done in a couple of ways.


Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication

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