Section 4

Correcting
the Compass

4.3

TVMDC

Magnetic and compass courses, headings and bearings must be converted to true before plotting them on a chart. Similarly, a true or magnetic reading must be converted to a compass reading to give to the helmsperson following the steering compass.

Lesson 3.2 presented the conversion to and from true and magnetic. The method was to apply the mnemonic Variation West, Magnetic Best; Variation East, Magnetic Least and to use the table:

T V M

where T represents true, V - variation, and M - magnetic. Now, D for deviation and C for compass can be added to this table. The result is:

T V M D C

The following mnemonic may be helpful to remember this:

TV Makes Dull Company

Converting True to Compass

To convert a true heading to a compass heading, the true must be converted first to magnetic with the local variation and then the magnetic to compass. If the true is 033° and the variation on the compass rose is 3°E, the magnetic heading will be "least" or lesser than the true and the variation subtracted:

T V M

D

C
033° 3°E 030°    

To convert the magnetic heading to a compass heading, the deviation must be added or subtracted. Remembering whether to add or subtract is helped by the mnemonic:

Deviation West, Compass Best; Deviation East, Compass Least.

The practice deviation table indicates that the deviation for a magnetic heading of 030° is 6°E. The deviation is east, therefore it's subtracted from magnetic to give the compass heading:

T V M

D

C
033° 3°E 030° 6°E 024°

To sail or motor a true heading of 033°, the helmsperson would steer a compass heading of 024°.

Converting Compass to True

To convert a compass course, heading or bearing to true, the process is reversed. The compass reading is known and the deviation can be found from the table. The variation can be taken from the compass rose.

In this example, a compass course of 172° is to be converted to true so it can be plotted on a chart. The deviation table indicates that for this compass course, the deviation is 3°W (using the deviation for 168°, the nearest compass reading in the table):

T V M D C
3°W 172°

The deviation is west, therefore the compass course will be "best", or greater than the magnetic. Thus 3° is subtracted from the compass course to get a magnetic course of 169°:

T V M  D C
169° 3°W 172°

Assume the chart shows a variation of 8°W. So 8° is subtracted from the magnetic course to arrive at the true course, that is 161°:

T V M  D C
161° 8°W 169° 3°W 172°

Memory Aids

To remember when to add or subtract in a conversion, use the mnemonics:

Variation West, Magnetic Best; Variation East, Magnetic Least

and:

Deviation West, Compass Best; Deviation East, Compass Least

 

Or use: TV Makes Dull Company Saturday Evening for T V M D C S E where S in "Saturday" stands for subtract, and the E in "evening" for east. East variation and deviation are subtracted when converting from left to right, or true to compass. The opposite applies when working with west variation and deviation, or converting from right to left.

For example, given a true bearing of 022° and a variation of 4°E, the variation is subtracted from the true to give a magnetic bearing of 018°. Had the variation been 4°W, it would have been added to give 026°.

 

The Sail Canada Basic Coastal Navigation Standard
9. Convert courses, headings and bearings between true, magnetic, and compass.

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