3_2 The Compass

Section 3 The Navigator's Compass

3.2

Magnetic North and Variation

It's said that a compass points north. That isn't exactly true. The magnet in a compass aligns itself with the magnetic field of the planet. This field flows between the earth's two magnetic poles, the northern of which is the magnetic north pole. The north geographic pole is the axial pole centered at the top of the planet where the meridians of longitude converge. This is true north.

The magnetic north pole is located in the Canadian arctic about 450 miles from the north pole. From most places on earth, true north and magnetic north aren't in the same direction. This angular difference between the direction of the two poles is variation and is expressed in degrees.

Variation and Location

The amount of variation changes with location as does its direction east or west. In Vancouver, for example, magnetic north is to the east of true north and the difference is 23°. The variation is therefore 23°E.

In Ottawa, magnetic north is west of true north by 14°and the variation is 14°W. The variation in Chicago is 0°. From there true north is in line with magnetic north. Variation is rarely more than 23° anywhere in lower latitudes.

Variation and
Annual Change

The variation for a charted area is shown inside the chart's compass rose. It's usually written along the arrow pointing to magnetic north. It's expressed as degrees east or west.

This illustration represents the difference in direction between true north and magnetic north. This difference is variation and it changes in amount and direction depending on location.

Unlike true north, the magnetic pole slowly changes position along a predicted path. As a result, for most locations variation changes slightly from year to year. This annual change, or drift, can be predicted and it too is stated inside the compass rose. Annual change in variation is in minutes east or west per year. If you're using an older chart it may be necessary to update the variation by adding or subtracting the annual change.

If the direction of the annual change is the same as the variation, the annual change is added to the variation; subtracted if it's opposite. Variation is rounded to the nearest degree. For example, if variation is 9°W in 1994 and the annual change is 4'W, by 2004 the variation will have changed by 40'W (four minutes per year for ten years). So the variation should now be considered to be 10°W.

Magnetic versus True

A course, heading or bearing taken from a hand-held compass is a magnetic reading. And when navigating you have to be able to convert magnetic to true.

In the conversion between true and magnetic, the variation, as indicated in the compass rose, is added or subtracted depending on whether the variation is east or west. Visualizing the apparent locations of the two poles is one way to decide whether to add or subtract the variation. Logic plays a part in this reasoning. Another, perhaps simpler way, is to apply this short mnemonic:

Variation West, Magnetic Best; Variation East, Magnetic Least

Use this mnemonic to remember that with variation west, the magnetic direction is going to be "best" or greater than true. Variation is added to true to arrive at magnetic, or subtracted from magnetic to get true. With variation east, magnetic will be "least" or less than true.

Converting T-V-M

The true-to-variation-to-magnetic conversion is abbreviated as T - V - M, where T is true, V is variation and M is magnetic. Here's the notation:

T V M

256° 4°E 252°

Given two variables, you can get the third:

T V M

114° 12°W ?

Since variation west means M is "best" or greater than T, then V is added to T to give 126° magnetic.

T V M

? 2°E 051°

Here, a magnetic reading of 051° is to be converted to true using a variation of 2°E. Variation is east, so M will be "least" or less than T by 2° : T must be 053°.

T V M

357° ? 001°

In this example variation is unknown. The boat may be on a course of 357° true and a hand-held compass reads 001° magnetic. The difference between T and M is 4° and M is "best". Apply "variation west, magnetic best" and you see that variation is 4°W.

"M" for Magnetic

When noting on a chart, entering in a log book, or plotting a magnetic heading or bearing, degrees are followed by the letter M, as in 134°M. This is important so that a magnetic reading isn't confused with a true reading. Directions are considered to be true unless indicated otherwise.

Forgetting to correct for variation can have disastrous consequences.

**Here, Wanderer has followed a course of 030°M, while the direction from the fairway buoy to the harbour is really 030° true.**

Section 3

The Navigator's Compass

3.2

Magnetic North and Variation

Variation and Location

Variation and Annual Change

Magnetic versus True

Converting T-V-M The true-to-variation-to-magnetic conversion is abbreviated as T - V - M, where T is true, V is variation and M is magnetic. Here's the notation: T V M 256° 4°E 252°

Converting T-V-M

Given two variables, you can get the third: T V M 114° 12°W ? Since variation west means M is "best" or greater than T, then V is added to T to give 126° magnetic.

T V M ? 2°E 051° Here, a magnetic reading of 051° is to be converted to true using a variation of 2°E. Variation is east, so M will be "least" or less than T by 2° : T must be 053°.

T V M 357° ? 001° In this example variation is unknown. The boat may be on a course of 357° true and a hand-held compass reads 001° magnetic. The difference between T and M is 4° and M is "best". Apply "variation west, magnetic best" and you see that variation is 4°W.

"M" for Magnetic