As you pass through the Chesapeake Bay Bridge and Tunnel, you take a bearing of 047°pgc along trestle C when it is in line. The helmsman reports the vessel's heading as 316° pgc and 329°psc. What is the deviation on that heading?
• Difference between per gyro compass (pgc) and per standard compass (psc) readings • How variation and deviation relate gyro, magnetic, and standard compass headings • Using the formula G-M = E (gyro error) and how deviation is the difference between magnetic and standard compass headings
• First, which heading (pgc or psc) is closest to the vessels true heading determined from the line of trestle C? • From the reported gyro and standard compass headings, what can you say about the gyro error (is gyro ahead or behind true)? • Once you know the true heading, how do you find the magnetic heading, and then the deviation on that heading?
• Make sure you clearly label every value as true, gyro, magnetic, or standard compass before doing any math • Verify the algebraic signs: remember "east is least, west is best" when applying variation and deviation corrections • Confirm that the final deviation angle and its E/W direction match the relationship between the magnetic and standard compass headings
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