True North:


True North is an imaginary straight line between you and the geographic North Pole (that theoretical dot at the top of the globe). This straight line is a great circle that passes through you and both the North and South Poles. It is called a Meridian of Longitude. Simple enough.

Magnetic North:


Here's where it gets a little tricky.

The Magnetic North Pole, that spot the needle or card on your compass points towards, is not located at the geographic North Pole. If you live in the Western Hemisphere, Magnetic North is actually located south of the geographic North Pole. As a result, and depending on where you are, there is almost always an angular difference between True North and the direction your compass is pointing.

This angular difference is called variation. To find out how much variation there is in your area, look at a local marine chart. There will be two (2) red circles on the chart, one inside the other. Each circle is marked in one degree increments from 000o to 359o. The 000o on the outer circle is pointing towards True North. The 000o on the inner circle is pointing toward Magnetic North.


The angular difference (variation) between these two directions, for that area, is printed in degrees and minutes, either East or West, in the middle of the two circles. (By the way, this handy dandy device is called a Compass Rose.) For example, if you are boating on Long Island sound, the variation is approximately 14o W(est). Therefore, if you want to proceed on a course of 090o True, you must add the 14o W variation to the 090o True. Your resultant Magnetic Course would be 104o.

Now, if you do your boating around Chicago, Illinois, the variation there is near 0o. However, if you are out west, the variation becomes easterly - and the angular difference increases as you go further west and north. Thus, if your variation is 14o E on a course of 090o True, you would subtract 14o from 090o, giving you a Magnetic Course of 076o.

The variation between True and Magnetic direction is always the same for all boats operating in a given area.

For those of you who are still with me, and because this isn't quite complicated enough, there is yet another factor to consider with a magnetic or fluxgate compass. Each compass may, or may not, be influenced by electrical currents or fields on your boat. This influence can cause additional compass error called deviation. Most deviation can be eliminated by adjusting the compensating magnets in your compass, in a process called "Swinging the Compass". (In the interest of not totally befuddling everyone, I will not cover that process at this time.)

http://powerboat.about.com/od/navigation/a/truenorth_wayup.h...