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.
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
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
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.
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