🔍 Key Concepts

• Compass error vs. variation vs. deviation • Use of charted ranges (two charted objects in line) to check compass performance • How a deviation table is constructed for specific headings

💭 Think About

• What does it mean when two charted fixed aids (light and aero beacon) visually appear exactly in line? How should their true bearing compare to your compass bearing if there were no errors? • If your vessel’s heading is 244° per standard magnetic compass, what does your deviation table tell you about errors on that heading, and how would you use a known range bearing to confirm or question that? • Under what circumstances would you suspect local magnetic disturbance instead of ordinary deviation or variation?

✅ Before You Answer

• Verify the true range bearing (from chart) between Block Island Southeast Point Light and Block Island Aero Beacon and compare it to the compass bearing observed (326°psc). • Check what the deviation table would predict for the ship’s head 244° and whether the observed alignment agrees with that prediction. • Confirm whether a single discrepancy on one heading with fixed, charted objects is enough to conclude local magnetic disturbance, or if it more likely indicates a normal compass error (variation + deviation).