You may have noticed, if you have looked at a nautical chart, that there were two rings on the compass rose. The outside ring is based on true north and the 000° direction points to the true North Pole. On the inside ring labeled “MAGNETIC” the 000° direction points in a different direction than the outside ring.
These basics are covered in the online Nautical Know How Basic Boating Safety Course at http://boatsafe.com/navigation .
The difference between True and Magnetic is called “Variation.” Variation is a naturally occurring disturbance caused by the magnetic north pole. Variation will change depending on where you are located on the earth. If you are operating on the West Coast of the US you will have east variation because the magnetic north pole is located east of you. On the East Coast of the US you have west variation because the magnetic north pole is west of your location.
Another error which may show up in your magnetic compass is called “Deviation.” Magnetic compasses are also affected by magnetic fields on the vessel itself. Just put some sort of ferrous metal such as a screw driver next to your compass and you will see an example of this error.
Significant errors of deviation could be caused by metal tanks, your engine, metallic objects near your compass or even electromagnetic interference caused by your radio.
To safely and accurately navigate from one position to another it is important to be able to correct for both “variation” and “deviation” and be able to steer a course by your magnetic compass.
If you want to learn more about the magnetic compass and how to calculate TVMDC…READ MORE.