OUPV / Six Pack Captains License Course

PLOTTING - Deviation



Deviation is the magnetic influences affecting your vessel. These change with your heading. Deviation can be affected by steel tools placed near the compass or other electronic equipment (i.e. wearing a nylon shirt with static electricity, hand-held VHF or cell phone). If your compass is mounted too close to the steel core of the steering wheel it can cause excessive deviation. Mariners often make a deviation card using incremental headings of 30 degrees around a complete compass rose. Together, variation and deviation equals compass error. Again, it is important to note whether it is east or west along with the numerical value.

To calculate the deviation of your compass you can use the phrase “True (T) Virgins (V) Make (M) Devine (D) Companions (C). Write the first letter of each term in a vertical line. Place “East” on top and “West” on the bottom as shown below:

East –

West +

T  True North (outer ring of the compass rose)
V  Variation (found on the compass rose of the chart for that area)
M  Magnetic North (inner ring of the compass rose)
D  Deviation (magnetic influences on your boat.  This is what you are solving for)
C  Compass (PSC – Per Standard Compass) Compass reading on your vessel

Now draw a downward arrow between East and West, placing an arrowhead by the West. This indicates the direction used to solve the problem. Place a + by the arrowhead (or West) and a – by the other end (or East). In this problem, anything East will be subtracted, anything west will be added. 

Here is a sample problem:

According to your chart, your true course is 113 degrees and your compass reads 125 degrees PSC (per standard compass). Your chart compass rose indicates the variation in your area is 14 degrees west. What is your deviation? You would solve as follows:

East –

West +

T  113 degrees
V  014 degrees West
M 127 degrees
D 002 degrees East
C 125 degrees PSC

(113 + 14 = 127   M = 127 degrees)

(To go from 127 degrees to 125 degrees you must subtract 2 degrees. Since east is -, deviation is east)

To find the deviation of your vessel, the proper procedure is to line up two fixed objects found on a chart to form a range. Take your vessel’s compass heading (PSC) and put it in the TVMDC formula at the bottom beside the C. Using your parallel rulers and dividers and your chart, find the true heading of that range. Place the variation from your compass rose next to the V. Make sure you place the + by the west and – by the east as well as the arrow, then solve for your deviation. The video will demonstrate how this is done.

  1. You are North of Stratford Shoal Middle Ground Light and you line up that light with Old Field Point Light. Your compass bears 209 degrees PSC. What is your deviation?
  2. If your compass bears 204 degrees, what is your deviation?
  3. If your compass bears 200 degrees, what is your deviation?

Practical Exercise: Compass error is variation and deviation together. Using this information, solve the following problems: