As you transit the Chesapeake Bay Bridge and Tunnel, you take a gyro bearing of trestle C when it is in line. The gyro bearing was 050°. At that time, the helmsman noted that he was heading 139°pgc and 146° per standard magnetic compass. What is the deviation?
• Gyro error vs compass deviation • Relationship between gyro compass, per standard magnetic compass, and true/gyro bearings • Sign convention: when is deviation labeled East vs West
• Compare the gyro bearing of the trestle with what the heading should be when the trestle is exactly in line. What does that tell you about gyro error? • Once you know the gyro error, convert between per gyro compass (pgc) and per standard magnetic compass (psc). Which way do you move to go from gyro to magnetic? • From the difference between magnetic and compass (psc) headings, decide if the compass is deflected East or West.
• Be clear on which value is pgc (per gyro compass) and which is psc (per standard magnetic compass). • Make sure you keep the sequence straight: True ↔ Gyro ↔ Magnetic ↔ Compass (psc) and apply corrections in the right order. • Double-check your East/West deviation sign: remember the common memory aid (e.g., "East is least, West is best" for converting between True and Compass).
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