You swung ship and compared the magnetic compass against the gyro compass to find deviation. Gyro error is 2° E. The variation is 8° W. Find the deviation on a true heading of 258°.
PSC PGC 030.5° 020° 061.5° 050° 092.0° 080° 122.5° 110° 152.0° 140° 181.0° 170° 210.0° 200° 239.5° 230° 269.0° 260° 298.0° 290° 327.5° 320° 358.5° 350°
• Relationship between true, gyro, and magnetic headings when there is a known gyro error • How to get magnetic heading from true heading using variation (East is least, West is best) • How to use the swing table (PSC vs PGC) to interpolate deviation at the required heading
• First, convert the true heading of 258° to both true gyro and magnetic compass headings, being careful with the gyro error sign (2° E) and the variation (8° W) • Then, look at the table of PSC vs PGC and figure out which column corresponds to the heading system you just computed for the compass being swung • Think about where 258° falls between the tabulated values and how to estimate the deviation between those two nearby entries
• Be clear on whether gyro error East means True = Gyro + or - error; write out a sample equation to check your sign • Confirm whether the table is giving you per ship’s compass (PSC) vs per gyro compass (PGC) headings, and which heading you must enter with to find deviation on the ship’s compass • Verify that your final deviation sign (East or West) matches the direction of the difference between magnetic and compass headings: if compass reads higher than magnetic, deviation is West; if lower, deviation is East
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