To convert a true course to a compass course, you need to account for both magnetic variation and compass deviation.
Variation is the difference between true north and magnetic north, while deviation is the error introduced by the ship’s magnetic field.
The formula is: Compass Course = True Course + Variation + Deviation.
When calculating, remember that east variation is subtracted and west variation is added, and the same applies to deviation.
True Course : 060°
Magnatic Course : 078°
Variation 18° 2024 decreasing 30` E annually
To convert a true course into a magnetic course we need first assign a “ − ” to a western and a “ + ” to an eastern variation.
This is important because of the clockwise ↻ direction of the compass rose. As per the above image , the inner circle is turned 18° anticlockwise, hence −18°.
Now: mc = tc − var i.e 78° = 60 − (−18°), and also notice the equivalence of +342° clockwise: mc = 60° − (+342°) = −282° and + 360° = 78°. Meaning: to sail a true course of 60°, the helmsman has to steer a magnetic course of 78°.
Hope this helps your search. in case you have any question, drop then on the comment box below, we will direct you.
Don’t forget to subscribe to our newsletter and follow us on social media
