Navigation
On aircraft or vessels there are even three types of bearings: true, magnetic, and compass bearing. Compass error is divided into two parts, namely magnetic variation and magnetic deviation, the latter originating from magnetic properties of the vessel or aircraft. Variation and deviation are signed quantities. As discussed above, positive (easterly) variation indicates magnetic north being east of geographic north. Deviation is positive if a compass bearing mark points to the right of the related magnetic bearing.
If one knows compass bearing and wants to determine true bearing the following calculations apply:
- Compass bearing + deviation = magnetic bearing
- Magnetic bearing + variation = true bearing.
Or starting from a given true bearing, the related compass bearing is obtained by:
- True bearing − variation = Magnetic bearing
- Magnetic bearing − deviation = Compass bearing.
It is often combined with "West is Best, East is least"; that is to say, add W declinations when going True to Magnetic to Compass, and subtract E ones.
Another simple way of remembering which way to apply the correction for Continental USA is as follows: For locations east of the agonic line (zero declination), roughly east of the Mississippi: The magnetic bearing is always bigger. For locations west of the agonic line (zero declination), roughly west of the Mississippi: The magnetic bearing is always smaller.
Common abbreviations are:
- TC = true course;
- V = variation (of the Earth's magnetic field);
- MC = magnetic course (what the course would be in the absence of local declination);
- D = deviation caused by magnetic material (mostly iron and steel) on the vessel;
- CC = compass course.
Read more about this topic: Magnetic Declination