Wednesday, April 30, 2008

Compass (Magnetic North)

The Magnetic North Pole, that the needle or card on your compass points towards, is not located at the geographic North Pole.

Depending on your location, 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 your 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. This is called a Compass Rose. The variation between True and Magnetic direction is equally the same for all boats operating in a given area.Each compass may, or may not, be influenced by electrical currents or electrical fields on your boat. This influence can cause additional compass error called Deviation. Most deviation can be eliminated by adjusting your compass. This was talked about in a earlier blog.

Compass North:

When you need to convert True Direction to Compass Direction or vice versa, just set up the formula this way.

True 100°
Variation 10° W
Magnetic 110°
Deviation 5° E
Compass 105°

Remember when going from True to Compass, you add West Variation and Deviation and subtract East Variation and Deviation. The opposite is true if you are going from Compass to True.

If you are having problems with your Global Positioning System (GPS) and your compass don't forget you can program them to use either True direction or Magnetic direction. If your GPS is giving you True direction, while your compass is giving the Magnetic direction you will be off course.

Also don't forget the direction the boat is traveling through the water (heading) can be different from the direction it is moving over the face of the earth, due to wind and current.