On 22 February your 2045 ZT position is LAT 33°19'N, LONG 52°06'W. You observe Polaris bearing 358.1°pgc. At the time of the observation the helmsman noted that he was heading 048°pgc and 065°psc. The variation is 19°W. What is the deviation for that heading?
• Compass error is the algebraic sum of variation and deviation • Using a known true bearing (from a celestial body like Polaris) to find the compass error on that heading • Relationship between psc (per standard compass), pgc (per gyro compass), and true bearings
• First, think about what the true bearing of Polaris should be when you are in the Northern Hemisphere, and how that helps you find the compass error of the standard compass at that moment. • Use the observed bearing of Polaris (pgc), the ship’s head (pgc and psc), and the known variation to step-by-step work from compass to true, or from true back to compass. • Once you determine the total compass error on that heading, remember how to separate that into variation and deviation so you can isolate the deviation.
• Be clear on the sign convention: is variation west added or subtracted when going from true to compass, and how does that affect deviation? • Confirm that you are finding deviation for the ship’s standard compass heading 065°psc, not for the gyro heading. • Double-check the direction (E or W) of your final deviation by seeing whether the compass is reading higher or lower than the corresponding magnetic direction.
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