Magnetic Compasses Demystified

By RUSS SWINNERTON

Remember at school, we learnt that the earth had a magnetic field, which we visualised as lines of magnetic force running from a magnetic south pole to a magnetic north pole? And we learnt that a compass was basically a magnet on a pivot that aligned with those lines of magnetic force, to point to magnetic north.

Simple enough so far. But when we finally try to use this compass thing as a navigational instrument, we find that a little more work is required. Because the magnetic north pole is in a different place to the true north pole, and because the earth’s magnetic field varies from place to place, magnetic north is not the same as true north.

The difference between true north and magnetic north in any particular place is called VARIATION, and is expressed in degrees and minutes east or west of true north.

(On topographic maps, you might also see this described as magnetic DECLINATION – but declination has a different meaning at sea, associated with astronomical navigation).

Variation varies from place to place, and changes with time. Your nautical chart will tell you the variation for the area covered by the chart, and the rate it’s changing.

There’s another source of difference between the compass reading and true north called DEVIATION. This is the term for the deflection of the compass needle caused by the ship’s magnetic field, and has nothing to do with the sexual orientation of the mariner!

Fibreglass or plastic kayaks are usually free from deviation – unless you put something ferrous (or electronic) in the forward hatch. Passing your dry bags full of stainless steel cooking pots or ghetto-blasters past the compass before stowing them will show you if they’re magnetic or not. If they are, better stow them aft. And don’t forget the effect of knives, phones, VHF radios etc stowed in your PFD if you’re using a hand-bearing compass.

The usual order of applying these errors is:

Compass reading – Deviation error – Magnetic reading – Variation error – True reading.

That’s C D M V T – easily remembered by the mnemonic CAN DEAD MEN VOTE TWICE. (And there are some naughty mnemonics I can provide in a plain brown-paper envelope…)

We kayakers don’t need to worry much about deviation, so we just deal with the CVT part of the equation. To convert a compass or magnetic bearing to a true bearing, you take the compass bearing and add east variation, or subtract west variation.

To go the other way, to convert a true course or heading to a magnetic heading, we subtract east and add west.

The easiest way to remember which way to apply the reading is to remember one of these four examples, and you can then figure out which way to apply it: Compass to True, Add East – easy to remember with the simple mnemonic CADET:
C V T
Compass ADd East True

When going the other way, from True to Compass, we subtract east. Trust me, the more you try it, the easier it gets.

(CADET always works in NSW, where variation is east everywhere. You will meet the other case, of west variation, if you paddle further afield. But you needn’t bother remembering CSUWT – apart from it not being a word, it’s easier to remember to apply the opposite sign – minus, instead of plus, when it’s west, rather than east.)

Let’s see that in practice: if we observe a magnetic bearing of 078M, with the variation at 12 degrees East — Compass to True, add East – this gives us a true bearing of 090T.

(We always give courses and bearings in three-digit form on a 360 degree circle – so the cardinal point east becomes 090, not 90).

And with the variation 12 degrees East, what magnetic course would we set to steer True North?

True to Compass, subtract East – the magnetic heading to steer is 348T.

Here are some examples to try [answers below]

  1. You measure your course to steer as 135T. The variation is 7W. What should be your magnetic heading?
  2. You observe the bearing of a lighthouse to be 262M. Variation is 11E. What is the true bearing to plot on your chart?
  3. On your chart, you measure a transit between two points of land as 320T. Variation is 12E. What magnetic heading would this be, if you lined up your kayak on the transit to check your compass?
  4. You’re steering 180M, and right ahead you see a large radio tower. Variation is 9W. What is its true bearing?

Caveat Emptor (which is Latin for ‘be careful when buying steering compasses on ebay’)

Steering compasses for kayaks coast a packet. Yet on ebay, you can see them on sale for a fraction of what it costs you to buy one from your local chandler. Be careful, unless the compass is balanced for your planned latitude, it may not work.

Lines of magnetic force don’t necessarily run horizontally. They rise or fall at an angle called ‘dip’, and like magnetic variation, dip also varies from place to place. In the southern hemisphere, the lines tend to rise upwards; in the northern, they tend downwards. Steering compasses usually have a card with a magnet attached, balanced on a pivot. The card needs to be balanced to counter the tendency of the magnet to tilt up or down to align with the dip of the lines of magnetic force.

For convenience, the compass makers divide the earth into several zones of similar dip (seven, for the Ritchie Kayaker compass). You can usually navigate through the zones adjacent to your own before the tilt of the card becomes a problem. NSW is in Zone 6, and Zones 5, 6 and 7 cover the east coast from Torres Strait to deep in the Southern Ocean.

So those ebay compasses from US suppliers are usually balanced for the US (Zone 1), with downward dip. Bring the compass into the southern hemisphere to our NSW latitude (Zone 6), and the counterweight adds to the effect of dip, instead of balancing it: their cards can tilt enough to make them inaccurate.

Answers to Sea Kayak Navigation questions:

  1. 142M
  2. 273T
  3. 308M
  4. 171T
Advertisements