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Deriving true heading from ships compass

Forum - Coastal Navigation - Deriving true heading from ships compass
Module Content - Sea Talks1
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Student
Conan Hom
Lexington, MA, USA
Crew Level 0
Status: Sailor
Qualified days: 8
Deriving true heading from ships compass
Posted 2018, Apr 23 03:14
If deviation is a function of the magnetic heading heading then if we know the true heading we can calculate the ships compass (true + variation +deviation). However if we have the ships compass reading and we know the variation but we wish to have the true heading, is it possible to find the true heading? I'm not so sure because we won't know the deviation (since we don't know the magnetic heading which is necessary to calculate the deviation). Is this a correct assessment?
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Grant Headifen
Reply 2018, Apr 23 14:59
To go from ships compass to magnetic heading you need to know the deviation. This is from a table associated to the individual ship.
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Student
Conan Hom
Lexington, MA, USA
Crew Level 0
Status: Sailor
Qualified days: 8
Reply 2018, Apr 23 22:16
Hi Grant, That's the issue: if we only have the ships compass to start with, then we can't get the deviation? So in practice what do people do if they only have the ships compass to get the magnetic and true headings if the deviation cannot be determined?
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Grant Headifen
Reply 2018, Apr 23 22:20
Deviation is never determined - is comes from a table supplied with the ship. Typically it is small. And can be checked with a handheld compass which is not subject to the metalic objects close by. I reality you will never really get this accurate to warrant it's use.
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Andrew Hecker
St Petersburg, FL, USA
Skipper Level II
Status: Big Time Sailor
Qualified days: 42
Reply 2023, Feb 24 00:18
Conan - if your boat doesn't have a deviation card, you can create one. Point the ship at different directions relative to either marked ranges or by using a hand compass (as Grant noted) and note the error. I'm a private pilot, and aircraft have a deviation card posted near the compass. Any time electronic equipment is changed in the cockpit, we "re-swing" the compass and generate a new deviation card. Many airports have a compass rose painted on the ground aligned to magnetic north. We drive the airplane to the middle of the compass rose, point the nose to the desired directions, and log the bearing reported by the ship's compass. If we're pointing E, and the compass says 092, we know we have to subtract 2 degrees for an accurate magnetic bearing. Best to swing the boat's compass on a calm day at slack tide lest thy hair turn white and cravings for rum increase! ha! Andy
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