Correcting Compass Error

Or how to tell what direction to point the boat or which way is home.

“Correcting” is converting a magnetic (M) direction to true (T). “Uncorrecting” is converting from true to a magnetic direction. “True” direction is relative to true north (north pole).

Variation is the failure of the compass to point to true north (90 Degrees North), this difference is due to the Magnetic North Pole being located in Canada. Variation is read from the compass rose on the chart. Variation is the same for all boats in the same area. Variation is the same for all directions in the same area.

Deviation is the failure of the compass to point to magnetic north due to local magnetic fields on a vessel.  These fields may be caused by the engine, instruments or other electronic equipment, power wiring, tools or any other magnetic objects.  Deviation is specific to an individual boat. Deviation changes with the boats heading. Deviation is read from the Deviation Table on the boat.

              ↓         T      ↑                True
     (-) E    ↓         V      ↑     (-) W	Variation
              ↓         M      ↑     	        Magnetic
     (+) W    ↓         D      ↑     (+) E	Deviation
              ↓         C      ↑                Compass

     Tele-Vision Makes Dull Children add Wonder.

     The heading must be uncorrected to provide a compass reading
     for the helmsman to steer.

Navigators Piloting and Charting Reference Card
Reference card with help for correcting and uncorrecting  a compass, speed / time formulas and other quick hints for navigators.


   True         347	237     160     070     ___     ___     ___     ___
   Variation	011E    014W    012W    014W    013W    011W    012E    014W
   Magnetic    	336     ___     ___     ___     135     196     310     068
   Deviation   	005E    007E    010W    021W    012E    018E    007E    010W
   Compass	331     ___     ___     ___     ___     ___     ___     ___

Examples 2:

   True         ___     ___     ___     ___     062     191     274     004
   Variation    011E    013W    012E    010W    ___     ___     ___     ___
   Magnetic     ___     ___     ___     ___     057     179     290     354
   Deviation    013E    009E    004W    006E    ___     ___     ___     ___
   Compass	025     114     265     358     045     185     276     010

HELP HELP HELP!!!!!!!!!!!

The skipper just ask me to give them a compass course to steer and when I ask for the “Deviation Table” for the boat they said there were no dirty magazines aboard the boat and furthermore their sex life was none of my business. And besides I can’t remember any of this stuff anyway. Now what the heck do I do??

The compass reading can in this case be taken directly from the chart using the magnetic Compass Rose if several cautions are used. The Deviation for a boat can be large enough that a course can have a significant error. This error can be enough that a course laid from Alki Point to the West Point buoy can be off enough that the boat would run aground east of the West Point Light House and not pass to the west of the buoy. The course can be checked by plotting a reciprocal and using a hand bearing compass to check from where you came from as well as the direction to the desired destination. Use the chart to keep track of the position of the boat on any other land marks. Using other types of position indicators such as GPS, water depth or radar.

Extreme caution needs to be used if visibility is impaired (such as fog or rain).


   True         347     237     160     070     122     185     322     054
   Variation    011E    014W    012W    014W    013W    011W    012E    014W
   Magnetic  	336     251     172     084     135     196     310     068
   Deviation    005E    007E    010W    021W    012E    018E    007E    010W
   Compass  	331     244     182     105     123     178     303     078

Answers 2:

   True         049     110     273     354     062     191     274     004
   Variation    011E    013W    012E    010W    005E    012E    016W    010E
   Magnetic  	038     123     261     364     057     179     290     354
   Deviation    013E    009E    004W    006E    012E    006W    014E    016W
   Compass  	025     114     265     358     045     185     276     010

Printer Friendly (pdf) Version 

Error Note: The first group next to last question the True course should be 322 NOT the 298 shown in the PDF version.  I have corrected the Table above and the PDF version.  09/14/2011 The new PDF file name is ( chart_02-b.pdf )  Sorry for the inconvenience.


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10 Responses to Correcting Compass Error

  1. Jim Ehnborn says:

    Great site. Learned about it in the AUXNAV class last night.

  2. Pingback: Piloting and Navigators Quick Reference | Boating Safety Tips, Tricks & Thoughts from Captnmike

  3. Andrew says:

    second to last answer on first group is wrong true course should be 322.

  4. Pingback: TVMDC – Practice – Correcting and un-correcting the compass | Boating Safety Tips, Tricks & Thoughts from Captnmike

  5. Genno Nuevo says:

    how to un correct the compass?

    • captnmike says:

      Un Correcting is converting from True to Magnetic – I always write the formula the same way every time then work up or down – Un correcting for me is working down on TVMDC and up to correct

  6. kenneth says:

    how to solve when true error is given

    • captnmike says:

      I am not sure what you mean by “True Error”

      Variation error is caused by Magnetic North not being in the same place as True North.

      Deviation error is caused by local magnetic fields on the boat.

      When I am solving compass problems, I write down the TVMDC formula with T at the top – then I fill in the information that I have, then solve for the blanks. Exactly like solving for the blanks in the exercise problems.

      I always solve in exactly the same way, working up or down for the answers. No shortcuts, no adding or subtracting just two numbers on a piece of scratch paper. Work the problems in the exact same way each time, this also makes it easy for someone else to check your work and you can also easily check your own work

  7. Anonymous says:


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