Marine 3 C Practice Test

Bearings used in navigation can be true (geographic) or magnetic (compass). The Earth’s magnetic field is not aligned with true north, so the offset at your location—magnetic variation or declination—must be applied to your bearings to use them correctly. To convert, apply the variation to the bearing you’re working with: if the variation is east, subtract it to get the magnetic bearing; if it is west, add it. For example, a true bearing of 030° with a variation of 10° East becomes a magnetic bearing of 020°. If the variation is 10° West, the magnetic bearing would be 040°. Variation changes with location and over time, so check current values on charts and update your calculations accordingly. Note that this is different from compass deviation, which is caused by the vessel itself.