How does magnetic declination affect compass bearings and how should you correct for it?

• A. It shifts magnetic north away from true north; adjust bearings by adding/subtracting the local declination to align with true north.
• B. It changes the color of the compass needle.
• C. It has no effect on bearings.
• D. You should always ignore it.

Answer: (A)

Magnetic declination is the angular difference between magnetic north (where the compass needle points) and true north (geographic north). Because your compass gives a bearing relative to magnetic north, that reading is offset from true north by the local declination. To use bearings on a map or for navigation with true north, you correct by adding or subtracting the local declination: if the declination is east, add it to the magnetic bearing to get the true bearing; if the declination is west, subtract it. For example, a magnetic bearing of 30 degrees with a declination of 10 degrees east becomes a true bearing of 40 degrees; with a 7 degrees west declination, it becomes 23 degrees. You can apply this correction by adjusting the compass itself (set the declination on the device) or by performing the add/subtract calculation to convert magnetic bearings to true bearings. Declination varies by location and changes slowly over time, so stay updated with current local values.