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PSC -> magnetic when you don' t know the magnetic (or don't assume PSC = magnetic)

Forum - Coastal Navigation - PSC -> magnetic when you don' t know the magnetic (or don't assume PSC = magnetic)
Module Content - Sea Talks3
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Student
Conan Hom
Lexington, MA, USA
Crew Level 0
Status: Sailor
Qualified days: 8
PSC -> magnetic when you don' t know the magnetic (or don't assume PSC = magnetic)
Posted 2022, Dec 25 12:35
"(1) You left Clinton Harbor 41 deg 15.5 min N and 72 deg 31 min W about an hour ago heading 180° per your ships compass (psc) " Technically the PSC is not the magnetic heading as PSC is the magnetic + deviation however we have used the PSC as the magnetic heading to determine the applicable deviation. I know the error is generally small but if wanted to technically get it accurate, how would you do it if you only had the PSC measurement to begin with?
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Andrew Hecker
St Petersburg, FL, USA
Skipper Level II
Status: Sailor
Qualified days: 42
Reply 2023, Feb 24 03:08
Not "technically" - PSC is not the magnetic heading. PSC adjusted for deviation is the magnetic heading. One needs to 'swing the compass' in advance in order to build the deviation table. Do not try to work nav problems backwards to get the deviation - that's not a real world option. We are given a deviation table for the problems. In the real world, one would either use the ship's deviation table, or they would create one using a hand bearing compass and/or charted ranges. For this problem, the actual course sailed out of the harbor isn't important - that's why we're plotting lines of bearing.
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