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Paddling Articles In the Same Boat

Lost Art

Navigating Without Batteries —
Why Good Compasses Go Wrong,
And What to Do About It

By Farwell Forrest
farwell@paddling.net

July 27, 2004

Always trust your compass. That's the first rule of navigation. When instinct or intuition points one way, but your compass stubbornly points another, believe your compass. Instinct and intuition are notoriously fickle guides, after all. Every year the woods echo with the panicky shouts of folks whose "infallible bump of direction" led them astray — just this once, you understand. It's never let them down before.

Happily, the magnetic needle isn't prey to human error. It can't be deceived, and it never tires. It's as constant as the timeless tug of the magnetic poles. Or is it? There's a second rule of navigation, as well, and it's every bit as important as the first: Never trust your compass completely. Contradictory? Yes. But come to think of it, the tug of the earth's magnetic poles isn't exactly timeless, either. Let's keep things simple, though. While the magnetic poles may wander a bit, any changes are likely to be small, at least within the compass of our lifetimes. Why, then, should we ever doubt the accuracy of the north-pointing needle?

Here's one reason. The next time you get ready to leave on a trip, take an inventory of your gear. What do you find? Sleeping bag, tarp or tent, food, spare clothing, paddles, life jacket.… No problem with any of these, is there? But keep looking. You have a knife, don't you? Or maybe two. And a flashlight. Spare batteries. A portable stove. Fishing lures. Spinning reel. Even a radio. And while you're at it, turn out your pockets. If yours are anything like mine, they accumulate a lot of stuff: pocket-watch (sometimes), pen-knife (always), keys, even a steel tape-measure.

Now take a look at your boat. Let's say you're a canoeist. Are your boat's seats and thwarts held in place with bolts? Are the ash gunwales and decks screwed on? No? You say you have plastic gunwales, instead, fastened with aluminum pop-rivets? OK. Any chance there's a steel reinforcing bar under that plastic? Better check.

Or are you a kayaker? Then look at the cables that work your rudder. Are they steel? Probably. And what about the clamps on your boat's waterproof hatches? The D-rings on your decks? The bolts on your foot-braces?

You get the point, I'm sure. In your boat or on the trail, you're accompanied by many bits and pieces of ferrous metal, every one of which is a tiny magnet. You're carrying magnetic poles around with you by the dozen. Why does this matter? To begin with, no magnetic compass really "points" to the earth's magnetic poles. Instead, it aligns itself with the prevailing magnetic field. In the absence of disturbing influences, that field will be the earth's field, and the compass will indeed "point" to the poles. But what happens when you surround your compass with small magnets, both in your gear and on your person? Then the prevailing magnetic field will be the vector sum of all the individual fields. Your compass won't know — or care — which one is the earth's field. And you won't know either. But you will care.

That's not all. Since the prevailing field is a vector sum of the individual fields, it will change in both orientation and intensity as your heading changes. I don't want to belabor this point. Vector arithmetic isn't my idea of a good time. But the conclusion deserves our attention. The difference between the correct magnetic heading — your heading relative to the north magnetic pole — on the one hand, and the heading indicated by your compass, on the other, is called "deviation." And unlike variation (the difference between true north and magnetic north, also known as "declination"), deviation usually isn't constant. It changes as your heading changes.

If hikers and canoeists keep their distance from electric transmission lines, large ore bodies, and metal fences, they seldom have to worry too much about deviation. A hiker can always drop his pack when he suspects that his compass is being confused by something in his kit. A canoeist can simply stand up. (At least he can in some conditions, and in some boats.) But kayakers — and canoeists who like to mount a compass to a thwart to guide them through an island labyrinth — do have cause for concern.

Does this describe you? Then you'll want to do something about the problem. First things first. Since the trouble often originates in your gear, put as much distance between it and your compass as you can. And just how much distance is enough? That depends. The only way to find out is to conduct a simple experiment. Place your deck compass on a wooden table or plastic bench. Stand beside the compass. (Empty your pockets, and don't wear a belt with a steel buckle.) Now, taking one item at a time in your hand, hold each piece of gear that could affect the compass out at arm's length. Is the compass card quiet? Then bring the item closer to the compass until the card starts to drift or the item touches the compass housing. Repeat this several times, approaching the compass from a different direction each time. Slow and steady does it. If the card never moves, even when an item is touching the housing, you don't need to worry about that particular piece of gear. If the card does drift or twitch, though, no matter how slight the movement, make a note of the distance at which you saw the first tremor. That distance — and a little bit more — determines how much distance is enough. (If the card trembles even when a piece of gear is held out at arm's length, you'll have to walk toward the compass from further away, while a friend watches for the first sign of movement.)

Sound tedious? It is. But you may be surprised at what you learn. Many stainless steel knives and tools have no effect on a compass. That's good news. On the other hand, some non-ferrous metals send the card spinning merrily. The culprit in these cases is usually nickel plating. The old nickel-plated brass matchsafes have "confused" more than one woodsman over the years. The upshot? Don't guess which items in your gear may affect your compass. Experiment, instead. Then you'll know for sure.

Once you've worked your way through everything in your packs that could possibly make your compass spin, design a loading plan for your boat, putting the objects that caused the most trouble as far as possible from your deck compass. Canoeists have it easy here. Their boats permit greater leeway in stowing gear. Kayakers will have to use their ingenuity. And they may still fail.

How will you know if you've succeeded in distancing your compass from undesirable influences? Once again, you'll have to experiment. There's a time-honored technique for checking steering compasses for deviation. It's called "swinging the compass," and that's a pretty fair description. Begin by getting a second compass that you can read to within one degree. A hand bearing compass or an orienteering compass is ideal. This will be your reference, or sighting, compass. If there's any doubt in your mind about its accuracy, check it against several prominent landmarks, ones that you can easily identify on the topographic map for your area. If your sighting compass is not accurate within two degrees, choose another. (Be sure to make allowance for the prevailing variation.)

Are there no suitable landmarks near you? Then look to the heavens. Anywhere in the northern hemisphere, Polaris (the North Star) will fix true north to within one degree. Take a hand bearing compass or orienteering compass outside on a clear night and face north. Locate the Great and Little Bears (Big and Little Dipper, if you prefer). Now follow the Pointers to Polaris. (Give your eyes a chance to adjust to the dark. Polaris is a comparatively faint star, and some of its companions are fainter still.) Once you have the North Star in view — it will be as high in the sky as your latitude, right on the horizon at the Equator and directly overhead at the North Pole — take its (magnetic) bearing and adjust for variation. If the result is within one degree of 000 True, your compass is spot on. Want greater accuracy still? Then wait till an imaginary line connecting Mizar (the second star in the Great Bear's tail) and Polaris is vertical. Polaris now lies directly above (or below) the North Celestial Pole. But don't plan on doing this in summer. By the time it's dark enough to see Polaris, the Bears have already swung too far round. You'll have to catch them in spring or fall.

Catching a Bear by the Tail

When you've satisfied yourself that your sighting compass is accurate, you're ready to start. Sailors have to swing their steering compasses while their boat is afloat. Paddlers will usually find it more convenient to check their compasses ashore, provided that there are no overhead power lines or nearby metal fences, and that the family car is parked far enough away. A grassy knoll or narrow, sandy spit is ideal. Having selected your site, aim your empty, unrigged boat due north, using your deck compass. (Your heading will be 000 Compass, or 000C, for short.) Next, pick a reference point on the horizon corresponding to this heading, and determine the bearing of that point using your second compass. To avoid any possibility of magnetic interference, keep your sighting compass away from your deck compass. How far away? A couple of feet should be enough. To be certain, though, you'll have to experiment. If you prefer, you can stand ten feet or so behind your boat and take the bearing by aiming along the keel line. This will also be necessary if you can't find a suitable reference point on the horizon.

Now that you've determined the magnetic bearing corresponding to a heading of 000C, record the bearing in a notebook. To distinguish it from the heading, label it as so many degrees Magnetic (000M, say). Then repeat the process for every other cardinal and intercardinal heading, as determined by your deck compass: Northeast (045C), East (090C), Southeast (135C), South (180C), Southwest (225C), West (270C), and Northwest (315C). Be sure to record the corresponding magnetic bearings for each heading. When you've finished, examine the table you've just compiled. If the difference between heading by deck compass and magnetic bearing by sighting compass (deviation, in other words) is zero on all headings, load and rig your boat and prepare to repeat the process. But what if the deviation isn't zero? If it's the same no matter which way your boat is headed, you've probably mounted your deck compass improperly. The lubber line must be parallel to your boat's centerline — ideally, in a small boat like a canoe or kayak, it should be mounted directly above the keel line. If it isn't at least parallel, you've introduced a systematic bias on all headings. The solution is simple. Mount the compass properly, with the lubber line running fore and aft. (It's a very good idea to check your deck compass's alignment regularly. It's essential to do so if you remove it for any reason and then replace it.)

Contrariwise, if the deviation varies with your heading, the likely explanation is magnetic interference, perhaps from rudder cables or other hardware. Whatever the cause, you're ready to proceed. Load and rig your boat, and get dressed in your paddling kit. Then get in the loaded boat and shove it round until it's once again headed due north by your deck compass. (CAUTION! Your boat is high and dry. It's not supported by water. Thermoplastic kayaks and canoes will usually stand the strain, but fiberglass boats may not. Chocking the bilge with foam wedges can help, however, as can a thick, springy turf. And a piece of poly tarp under the keel will protect the hull from scratches. If you have any doubts about your boat's strength, swing your compass while floating in calm, shallow water, running bow and stern lines to a light anchor to control the angle.) As before, have a friend stand directly behind you with your sighting compass, and have him record magnetic bearings for each of the eight cardinal and intercardinal headings in turn, sighting along your keel line. Then compare headings and corresponding bearings. Is the deviation zero on all points? Excellent. As long as you stick to your loading plan — and don't get a bigger knife! — you have nothing to worry about.

Too good to be true? Probably. All too often, you'll find that once you've loaded your boat, your compass can no longer be trusted. Compass and magnetic bearings are no longer the same on every heading. What's the next step? Sailor's compasses often incorporate built-in compensating magnets to eliminate deviation. If yours does, follow the manufacturer's instructions. Few kayak compasses have such compensators, however. You'll probably have to prepare a deviation table, instead. Here's how. Compare the compass heading to the magnetic bearing at every point. If the magnetic bearing is the larger of the two — say that your sighting compass indicated 005M when your boat was headed 000C by the deck compass, or 095M when your boat was headed 090C — record the difference as so many degrees east deviation (5 degrees east deviation in these two examples). If, on the other hand, the magnetic bearing is less than the compass heading (remember that a heading of 000 is equivalent to 360 degrees, so that a bearing of 355M is less than 000C), record the deviation as so many degrees west. Then, once you've finished doing the arithmetic, arrange the results in tabular form, giving the deviation for each compass heading. That's all there is to it.

Well, maybe it's not quite all. Now that you have your table, you need to know how to use it. Here's where we meet an old friend again: CADET. To go from compass to true, just add east (and subtract west). Deviation or variation, it makes no difference. The procedure is the same for both. Take your compass heading. Add (or subtract) the deviation indicated in your table, using the value for the nearest listed heading among the eight cardinal and intercardinal points. This gives you the corrected magnetic heading. Then add (or subtract) variation to get your true heading.

Are you going from chart or map to compass, instead? Just reverse the process. Subtract east deviation or variation and add west. This is a good time to recall one of the advantages of nautical charts — you can work directly in degrees magnetic. If you take your course from the magnetic scale on the chart's compass rose, you needn't concern yourself with variation. You still have to "uncorrect" magnetic courses for deviation, however. If you don't, your deck compass will prove a false friend indeed.

It sounds complicated, and it is — at first. But practice makes perfect. (The navigation courses taught by US and Canadian Coast Guard Auxiliary units and Power Squadrons offer plenty of drill.) Soon you'll be correcting for deviation without a thought. Never forget this one thing, though: if you change your loading plan — or even if you get a larger knife — your deviation table may no longer be valid. You'll need to swing your compass again. A pain? Yes. But it's worth a little trouble to stay on course, isn't it?

A compass is the small-boat navigator's most important tool. Trust it, by all means. But never let your trust become absolute. Your compass doesn't have a brain. You do. And you're the captain of the ship. 'Nuff said?

Copyright 2004 by Verloren Hoop Productions. All rights reserved.
















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