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Compass Variation and Deviation
All information on a chart, including your own plotting, is related to TRUE North.
Thus all bearings on a chart are related to TRUE NORTH. Compasses point to MAGNETIC NORTH, which varies from True North by an error called VARIATION.
Compasses also are subject to their own errors; this is called DEVIATION.
COMPASS BEARING = TRUE BEARING +/ MAGNETIC VARIATION +/ COMPASS DEVIATION
Magnetic Variation is due to the differing positions of the Geographic (True) North Pole and the Magnetic North Pole. The boat's compass and a hand bearing compass point to the Magnetic Pole, but all bearings on charts are related to the Geographic Pole (True North).
Around UK coasts, variation is around 4° West to 7° West. In other parts of the world it can reach more than 50° (East or West). Because the Magnetic North Pole moves, the local variation changes from year to year. The compass rose on the chart gives the annual change, around 7’ East per year around the UK.
(For an 1870 version of Variation at 18° , see Chart of 1870 )
We therefore have to convert our True bearings into Magnetic, then Compass, to go from chart to compass and vice versa.
The simple way to remember is that whenever the Variation is West, the Magnetic bearing will be greater than True (see diagram above), ie you have to add Variation to True bearing to get Magnetic (or subtract Variation from Magnetic to get True).
“Variation West, Magnetic Best”
“Variation East, Magnetic Least”
If you use a plotter correctly (see conventions ) it will do the sums for you.
Boats themselves can also affect compasses due to their construction, steel engines, keel, ferromagnetism, electromagnetism, (loudspeakers, mobile phones, hand held VHF) and deck cargo on large ships. This effect is called Deviation, and will vary with the heading of the boat. You can plot a graph of the effect of change in boat heading on the Deviation, this is called a Deviation Curve. Deviation can be measured by ‘swinging the compass’, that is, checking the compass reading when the boat is on a known heading, round a 360° series of headings, and plotting the resultant errors. You can then read off Deviation and apply this to the boat's compass heading to get a Magnetic course, or vice versa, apply it to a Magnetic heading to obtain a Compass Course
Compass adjustment  Compass Deviation Card
ERROR degrees 

HEADING OF BOAT 
On commercial vessels it is so important that checks are carried out on every watch:
“The standard compass error is determined at least once per watch and, when possible, after any major alteration of course; the standard and gyro compasses are frequently compared, and repeaters are synchronised with the master compass.”
Compass Compensation  a good article. There is an example of a Deviation curve at the back of your RYA Training Almanac. You need to be aware of deviation and its causes.
Swinging the Compass
1) Locate two transit objects (objects in line) and determine
their true bearing on the chart
(2) Secure the boat at anchor so that these transits are exactly aligned. Swing
the boat's head until it is pointing due north.
(3) Read off the transit bearing on the compass and apply the variation.
(4) The difference between this result and the true transit bearing is the
deviation on this heading. If the true bearing is greater, the deviation is
named east; if it is the lesser, the deviation is named west.
(5) Repeat the procedure taking transit bearings on each of the cardinal points.
(6) From the results, make up a deviation card
Further errors are caused by boat heel, and the difficulty of reading when on a moving platform. An electronic compass can be set up to remove the effect of deviation, but it is still a magnetic compass.
Hand bearing compasses usually show very small deviation effects which can be ignored if you use them away from a magnetic area.
Deviation can also be East or West, but the same logic applies:
“Deviation West, Compass Best"
Summary and Examples
Examples:
True bearing
= 273° (T)
Var
= 5° W
Magnetic bearing = 278° (M)
True bearing
= 008° (T)
Var
= 13° E
Magnetic bearing = 355° (M)
Magnetic bearing = 273° (M)
Var
=
7° W
True bearing
= 266° (T)
Compass bearing
= 177°
(C)
Deviation
=
5°
E
Magnetic bearing = 182°
(M)
More variation and deviation examples: complete the table below:

True 
Variation 
Magnetic 
Deviation 
Compass 
1 
114 
4 W 

5 E 

2 

7 W 
309 

314 
3 
050 
6 E 


044 
4 

2 E 
248 
2 W 

5 
156 

148 
5 E 

6 
358 
9 W 


010 
7 
004 

358 
4 W 

8 

6 W 

3 E 
191 
Answers

True 
Variation 
Magnetic 
Deviation 
Compass 
1 
114 
4 W 
118 
5 E 
113 
2 
302 
7 W 
309 
5 W 
314 
3 
050 
6 E 
044 
0 
044 
4 
250 
2 E 
248 
2 W 
250 
5 
156 
8 E 
148 
5 E 
143 
6 
358 
9 W 
007 
3 W 
010 
7 
004 
6 E 
358 
4 W 
002 
8 
188 
6 W 
194 
3 E 
191 
Posted 4th March 2011