Learn About Deviation

There are three main causes of Compass Error

  • First is Variation which is the difference between the geographical north direction and the direction your compass actually points to, i.e. True North Pole and the Magnetic North Pole. The Variation is noted on every chart  in the Compass Rose.  Variation varies a lot all over the world, but it is always the same for every vessel in the same geographic area.
  • Second is Lubber Line Error, which is caused by not aligning your compass exactly to the physical center line of your vessel. It is very important that your compass actually points the exact same direction your vessel is headed.
  • The third error is Deviation, the purpose of this article. Deviation is the error which is particular to your vessel and is caused by magnetism aboard your vessel.  There are many causes for this magnetism, such as speakers, wiper motors, bad wiring, and of course your big cast irron engine.  The first thing a professional compass adjuster will do is look for and eliminate as much of this error as possible, such as suggesting the best location for your compass and/or your electronics.

The remaining deviation must be corrected for, and this is the main job of the professional compass adjuster.. At first this seems a simple process, just add equal and opposite magnetism near the compass to cancel out the offending magnetism.

 (For those of you who want to know more, the effect of one magnet on another is inversely proportional to the square of the distance they are moved away from each other.  In other words, moving one magnet away from another magnet 3 " reduces the effect they have on each other by a factor of 9, the square of 3.  i.e.: Putting very small magnets close to your compass will cancel out the big magnet on your boat, the engine.)

 In theory, error is always equal and opposite on the cardinal headings, (Negative East error equals Positive West error), fixing East should fix West and fixing South should fix North.  But in reality, theory is not what we find in the real world.  The art of compass adjusting is dealing with removing as much deviation as is possible on all headings, which almost always will leave you with some remaining compass error.  You will use the deviation table provided by your compass adjuster to correct the difference between what your compass reads and your intended heading.

Every navigator should memorize the following list in order from top to bottom:

  • T rue Heading
  • V ariation Value
  • M agnetic Heading
  • D eviation Value
  • C ompass Heading

Going down the list from True to Compass, ADD WEST Error and SUBTRACT EAST error.  

Going up the list from Compass to True you must SUBTRACT WEST error and ADD EAST error.  

Of course, you get the value for Variation from your chart on the compass rose and you get the value for Deviation from your Deviation Card.

As most electronics, (Plotters, GPS etc.), usually display TRUE headings you must always do the above math to correct for your compass error.

To lean more about how I accomplish your compass adjustment, see myCompass Adjustment Page