Compass declination is the angle between True North and Magnetic North at a specific place on Earth. It matters because a compass points toward Magnetic North, while most maps are drawn using True North or Grid North. If you ignore declination, your bearing can send you far off your intended route.
Good navigation means knowing which north your map and compass are using before you travel.
Key Facts
- Declination = angle between True North and Magnetic North at your location.
- True North points to the geographic North Pole, while Magnetic North points to Earth's magnetic field direction.
- Grid North is the north direction of vertical grid lines on a map, and it may differ slightly from True North.
- For an east declination, convert true bearing to magnetic bearing with Magnetic = True - Declination.
- For a west declination, convert true bearing to magnetic bearing with Magnetic = True + Declination.
- Back bearing = Bearing + 180° if the result is less than 180°, or Bearing - 180° if the result is 180° or more.
Vocabulary
- Compass declination
- Compass declination is the angle difference between True North and Magnetic North at a given location.
- True North
- True North is the direction along Earth's surface toward the geographic North Pole.
- Magnetic North
- Magnetic North is the direction a compass needle points because of Earth's magnetic field.
- Grid North
- Grid North is the north direction shown by the vertical grid lines printed on a map.
- Bearing
- A bearing is a direction measured in degrees clockwise from north.
Common Mistakes to Avoid
- Using a compass bearing directly on a map without correcting for declination is wrong because the compass and map may use different north references.
- Adding east declination when converting from true to magnetic is wrong because east declination is subtracted for that conversion.
- Treating Grid North and True North as always identical is wrong because map grid lines can be slightly rotated compared with geographic north.
- Using an old declination value is wrong because Magnetic North moves over time, so the correct angle can change from the value printed on an older map.
Practice Questions
- 1 A map gives a true bearing of 075° to a campsite. The local declination is 10° east. What magnetic bearing should you set on your compass?
- 2 You follow a magnetic bearing of 240° in an area with 6° west declination. What is the true bearing on the map?
- 3 A hiker lines up a map with a compass but does not account for a 12° east declination. Explain how this could affect the hiker's route and what correction should be made before traveling.