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Paddling Articles In the Same Boat

Lost Art

Navigating Without Batteries —
A Tale of Two Norths

By Farwell Forrest
farwell@paddling.net

May 11, 2004

A map or chart without a north arrow is little more than a pretty picture. It's nearly useless for navigation or route-finding. But a north arrow or compass rose printed in the margin isn't enough by itself. You still need to "orient" your map, to match north on the map with north on the ground. On a clear night, you can do this by turning your eyes to the heavens, though only if you know where to find the star you need. During the day, you can compare the map with what you see around you — if the local terrain is sufficiently distinctive, and if you can correctly interpret the shorthand symbols used by the mapmaker as he struggled to compress three dimensions into two.

But what can you do when you want to know where north is right now? Wouldn't it be wonderful if you could carry north in your pocket, so to speak, and then bring it out anywhere you need to get your bearings, at any time of day or night? Well, I've got some good news for you. You can. And the tool you need isn't exactly new. The Chinese were exploring its possibilities back when St. Peter booked passage to Rome.

What is this marvel of time-tested technology called? The magnetic compass, of course. There's probably at least one compass in every paddler's kit bag. Nonetheless, a surprising number of canoeists and kayakers get "confused" — that's the navigator's euphemism for "lost" — every year, even on familiar waters. How can this happen? Aren't a map (or chart) and compass all you need to find your way? In a word: No. You have to know how to use them, too. And you need to understand their limitations. For one thing, a compass isn't a GPS receiver. A compass indicates direction. Period. It won't generate a Lat-Lon readout, or give you a position accurate to within 10 meters. Then again, a compass doesn't need batteries. Or does it? The engineers haven't been entirely idle in the two millennia since St. Peter boarded his ship. If you want a compass that does require batteries to work, you can have one. At a price. Just look in the catalogs for "flux-gate compasses."

We're talking about navigating without batteries, though, aren't we? So let's stick to the magnetic compass. Why don't paddlers make more (and better) use of it? One answer to this question can be found in an unhappy accident of history: the history of our planet. North — geographic, or "true" north, that is, often abbreviated TN — is defined by the earth's axis of rotation. North is the direction to the North Pole, in other words. Paddle due north long enough and you'll find yourself on top of the world, with nowhere to go but south. You'll be at the North Pole. So far, so good. But there's a catch. Except in certain places along more or less well defined "agonic" lines — one runs north through the Florida panhandle to the western tip of Lake Superior and continues on toward Hudson Bay; another traverses the Koryak Range east of Kamchatka, and then sweeps south and west, eventually crossing the narrow waist of Vietnam — the north end of a compass needle doesn't point toward the North Pole.

There are, it seems, two norths. One, true north, is the north established by the bearing of the North Pole. The second, "magnetic" north or MN, is the north toward which a compass needle tends. And the difference between the two? It's known to land navigators (and most canoeists) as "declination." Mariners, who already had another use for that word by the time Europeans recognized the existence of two norths, call it "variation," instead. By either name, it can be as little as zero — along the agonic lines already mentioned — or as much as 180 degrees (that's one-half of a full circle). There's worse to come. Not only does declination vary from place to place on the earth's surface, but it can lie in either direction of true north. The compass needle can point to the east of true (east declination), or it can point to the west (west declination). As if this weren't enough, declination isn't constant. It changes from one year to the next, sometimes by more than 10 minutes of angle (one-sixth of a degree) each year.

The Two Norths

It's no wonder that many paddlers leave their compasses in their kit bags. Still, it really isn't too hard to make sense of declination. Let's begin with a brief look at why it exists in the first place. A compass needle isn't tied to the North Pole by some supernatural sympathetic force. It's just a wafer of magnetized steel. And like all magnets, the needle is influenced — attracted or repelled — by other magnets and their associated fields. As it happens, the largest of these, as well as the most pervasive, is the magnetic field generated by the enormous "self-exciting dynamo" at the earth's molten core. Unfortunately, the north pole of this field (the magnetic pole) doesn't coincide with the geographic North Pole. And that's not all. The magnetic poles wander a bit from year to year. Every now and then, in fact, the earth's magnetic field does a complete about-face: magnetic north becomes magnetic south. The change is already overdue. It'll happen any millennium now. But that's another story.

Other magnetic fields also affect the needle of a compass. You should always assume that any iron or steel in your gear or boat will tug your compass off course. So will power lines, magnetic anomalies associated with ore bodies, and barbed wire fences, not to mention steel belt buckles and electronic watches. The solution? Distance. Your compass needs space. And just how much is enough? It depends. Experiment. You may have to take off your pack or rearrange the gear in your boat. If that doesn't do the trick — if your deck compass has a persistent error that can't be removed by juggling your gear and which can't be explained by some local magnetic anomaly (sailors call this error "deviation") — you may have no choice but to compile a card giving the deviation for each of the major compass headings. That, too, is a story for another time.

In any case, all the trouble with declination begins with the drunken wanderings of the earth's magnetic poles. It is, as I said earlier, an accident of history. Let's recap. The needle of a magnetic compass doesn't point toward the North Pole. Instead, in the absence of strong local fields (ax heads or power lines, say), it aligns itself with the earth's magnetic field, and — at least in most places — magnetic north does not coincide with true north.

Why does this matter? For one thing, map-makers usually orient the north-south axes of their maps and charts along the north-south axis established by the earth's geographic poles. If they didn't, they'd have to bring out new editions every few years, as the magnetic poles continue their tipsy dance around the earth's high latitudes. It's better to stick with the sober, steadfast geographic poles.

Of course, cartographers still have to rely on surveyors to plot the data points they use in making their maps, and those surveyors rely on magnetic compasses in the field. (They did until recently, at any rate.) So do navigators, even today. Obviously, some means of going from map to compass and back again is needed. And it exists. The key lies in the margin of any topographic map. That's where you'll find the declination diagram. (It's built into the compass rose on nautical charts.) This tells you at a glance the amount of the declination (or variation) in degrees, its direction (east or west of true north), and sometimes its rate of change. Why is the rate of change often omitted? Since most field bearings have an uncertainly of at least a couple of degrees — bearings taken while under way in a small boat in even a moderate sea will vary more than this, often a great deal more — small annual changes in declination don't matter much. If you're using a map that was printed more than ten years ago, however, it's a good idea to correct the declination for cumulative annual changes. Quads don't always provide this information, unfortunately, but most nautical charts do. It's labeled "Annual Increase" or "Annual Decrease." (WARNING! A ten-year-old chart will probably have to be corrected for other things, too. The locations of buoys and other aids to navigation may have changed, for example, or a hurricane may have rearranged the coastline. Printed maps and charts are static. But the earth and its waters aren't.)

Once you know the declination, you're nearly done. If you're going from map to field, and if the declination is west — that is, if magnetic north (MN) lies to the west of true north (TN) — just add the declination to the true bearing you obtained from the map. The result is the compass bearing you're looking for. In the case of east declination, subtract the declination from the true (map) bearing to get the compass bearing.

But what if you're going from field to map? Just turn things around. Subtract west declination from your compass bearing to get the true bearing. And if the declination's east? Then add it to the compass bearing before transferring it the map.

Confused? You're not alone. If, like me, you find yourself completely at sea when trying to remember this tangle of rules, you have two choices. With many orienteering compasses, you can simply "dial in" the declination. Then, so long as the declination doesn't change, the bearing you read (or set) on your compass will alway be the true bearing. No other adjustment for declination is needed. But the deck compasses used by kayakers don't permit this, and many paddlers — I'm one — always use compasses that lack a handy declination offset. So we're forced to fall back on one of the mnemonic catch-phrases invented to guide ocean navigators-in-training as they confront the mysteries of variation and deviation for the first time. Most of these, like "Timid Virgins Make Dull Company," betray their naval origins immediately, though one ("Can Dead Men Vote Twice?") sounds like it was coined in the old days of the Chicago Democratic machine, when the answer was invariably Yes. Entertaining as these are, however, canoeists and kayakers are probably best served by the prosaic "CADET." It stands for "To Compass ADd East Declination to Yield True." Everything follows from this. Try it and see for yourself.

A final caution is in order here: Don't think that you can cut corners and escape the need to add and subtract declination by extending the magnetic north (MN) arrow on a topographic map's declination diagram and then using this as your north reference. You can't. At least you can't count on being able to do so. The printed arrow often exaggerates the declination. On the other hand, the compass rose on a nautical chart incorporates an accurate magnetic scale. This makes working with a deck compass easier — provided that the chart is current and that you never confuse the magnetic and true scales.

We've begun to get our bearings, but we've still got a long way to go. Despite its two-thousand-year history, the magnetic compass retains its power to befuddle novice and expert alike. There's a lot more to say about this most fundamental of the navigator's tools, and we'll continue our exploration later. Till then, I'll leave you with the words of Alexander Pope:

Ask where's the North? at York, 'tis on the Tweed;
In Scotland, at the Orcades; and there,
At Greenland, Zembla, or the Lord knows where.

Pope's arctic geography is a bit muddled — that's no surprise; he wrote these lines in 1733, when the high latitudes were mostly unknown country — but he had the right idea nonetheless. If you begin by asking "Where's the North?" and then work out the answer, you'll never be confused for long.

Copyright 2004 by Verloren Hoop Productions. All rights reserved.






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