On 11 January your 0450 ZT position is LAT 38°42'N, LONG 14°16'W. You observe Polaris bearing 358.5°pgc. At the time of the observation the helmsman noted that he was heading 160°pgc and 173°psc. The variation is 9°W. What is the deviation for that heading?
• Compass error relationships between true, gyro, magnetic, and compass headings • Difference between psc (per standard compass) and pgc (per gyro compass) • How to apply variation and solve for deviation using T–G–M–C–A diagrams
• Start from the known gyro heading (160°pgc) and the known standard compass heading (173°psc). How do these relate to magnetic heading and deviation? • You know the local variation is 9°W. If you convert between magnetic and compass, where does deviation fit in the chain T–G–M–C–(A)? • Is the compass reading more east or more west than magnetic for this heading? What sign (E or W) of deviation does that correspond to?
• Be clear which instrument each value comes from: pgc = gyro, psc = standard/magnetic compass • Remember the rule: Variation is between True and Magnetic; Deviation is between Magnetic and Compass • Double-check the sign convention: whether an easterly error is added or subtracted in your T–G–M–C work, and match that to the multiple-choice options
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