Welcome back to the Art of Manliness Land Navigation Manual. In Part I, we covered how to properly use a compass, how to shoot your bearings, and the best map for navigating: the topo map. Before we can start actually navigating, which we’ll get to in Part III of this series, we still need to learn where we are in relation to our map — how to orient ourselves. So in this installment, we’ll go over the skills you need to do just that.
Adjusting Your Compass For Declination
Remember declination from Part I? Now that we’re putting compass to map, we need to adjust for it so that we can accurately orient and navigate.
Before we do that, let’s dig a bit deeper into declination. It will make it easier to understand why we adjust for declination in the first place.
As mentioned in Part I, true north and magnetic north aren’t the same. True north is at the very top of the earth, while magnetic north is currently off the coast of Greenland. The angle between true north and magnetic north is declination. But here’s where things get tricky: the angle of declination changes depending where you are on the earth.
Look at this map below:
See the line labeled the “agonic line”? When you’re on this line and are facing true north, magnetic north lines up perfectly, too. No declination exists and you don’t have to adjust your compass.
But let’s say you’re in the Muir Woods near San Francisco and you’re facing true north. Magnetic north would be slightly to your right, or east, by about 14°. If you point your compass towards true north, your compass needle is going to point a little to the east. Its declination would be written as 14E. Ueasy Military Compass... Buy New $18.85 (as of 02:10 EST - Details)
Let’s move to the opposite coast of the U.S. Let’s say you’re in the Adirondacks of New York. When you face true north, magnetic north is going to be left, or west, of you by about 14°. When you point your compass towards true north, your compass needle is going to point a little to the west. So its declination would be 14W.
Basically, in areas west of the agonic line, the compass needle will point somewhere to the east of true north; in areas east of the agonic line, the compass needle will point somewhere west of true north.
What happens if you don’t adjust your compass for declination? Let’s go back to our Muir Woods example. If we didn’t adjust for declination and we’re getting bearings for our destination from our map, we’ll find ourselves off by 14° or more when we start walking. That can make a huge difference!
To avoid that problem, let’s adjust our compass to take declination into account.
What if you don’t have a compass that allows you to adjust for declination? Well, you’re going to have to do some math with all your bearings to make sure you get a true bearing. UST Deluxe Map Compass... Buy New $4.61 (as of 05:30 EST - Details)
If your local declination is east of the agonic line, you’ll need to add the declination to the bearing on your compass; if your local declination is west of the agonic line, you’ll subtract the declination from the bearing on your compass.
So, if you’re in Muir Woods and your compass has a bearing of 180°, you’ll want to subtract 14°. So your true bearing would be 166°.
If you’re in the Adirondacks, you’ll want to add 14° so your true bearing would be 194°.
Another trick for declination when your compass can’t adjust for it is to create your own declination arrow out of masking tape and tape it beneath your compass under the correct declination degree hash (This will only work if your compass has a clear base). You’ll line up your needle with the tape arrow instead of the arrow on your compass.
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