Editing Magnetic declination (section)

== Navigation ==
On aircraft or vessels there are three types of [[Bearing (navigation)|bearing]]: 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 that magnetic north is east of geographic north. Likewise, positive (easterly) ''deviation'' indicates that the compass needle is east of magnetic north.<ref>{{cite web |last1=Willemsen |first1=Diederik |title=Compass navigation |url=https://www.sailingissues.com/navcourse3.html |website=SailingIssues |access-date=4 January 2020}}</ref>

Compass, magnetic and true bearings are related by:

<math display=block>\begin{aligned}
T &= M + V\\
M &= C + D
\end{aligned}</math>

The general equation relating compass and true bearings is

<math display=block>T = C + D + V</math>

Where:

* <math>C</math> is Compass bearing
* <math>M</math> is Magnetic bearing
* <math>T</math> is True bearing
* <math>V</math> is magnetic Variation
* <math>D</math> is compass Deviation
* <math>V<0, D<0</math> for westerly Variation and Deviation
* <math>V>0, D>0</math> for easterly Variation and Deviation
For example, if the compass reads 32°, the local magnetic variation is −5.5° (i.e. West) and the deviation is 0.5° (i.e. East), the true bearing will be:

<math display=block>T = 32^\circ + (-5.5^\circ) + 0.5^\circ = 27^\circ</math>

To calculate true bearing from compass bearing (and known deviation and variation):

*Compass bearing + deviation = magnetic bearing
*Magnetic bearing + variation = true bearing

To calculate compass bearing from true bearing (and known deviation and variation):

*True bearing - variation = Magnetic bearing 
*Magnetic bearing - deviation = Compass bearing

These rules are often combined with the mnemonic "West is best, East is least"; that is to say, add W declinations when going from True bearings to Magnetic bearings, and subtract E ones.

Another simple way to remember which way to apply the correction for continental USA is:
* 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 deviation);
*D  = deviation caused by magnetic material (mostly iron and steel) on the vessel;
*CC = compass course.

===Deviation===
[[Magnetic deviation]] is the angle from a given magnetic bearing to the related bearing mark of the compass. Deviation is positive if a compass bearing mark (e.g., compass north) is right of the related magnetic bearing (e.g., magnetic north) and vice versa. For example, if the boat is aligned to magnetic north and the compass' north mark points 3° more east, deviation is +3°. Deviation varies for every compass in the same location and depends on such factors as the magnetic field of the vessel, wristwatches, etc. The value also varies depending on the orientation of the boat. Magnets and/or iron masses can correct for deviation, so that a particular compass accurately displays magnetic bearings. More commonly, however, a correction card lists errors for the compass, which can then be compensated for arithmetically. Deviation must be added to compass bearing to obtain magnetic bearing.

===Air navigation===
Air navigation is based on magnetic directions thus it is necessary to periodically revise navigational aids to reflect the drift in magnetic declination over time. This requirement applies to [[VHF omnidirectional range|VOR]] beacons, [[runway]] numbering, [[airway (aviation)|airway]] labeling, and [[aircraft vectoring]] directions given by [[air traffic control]], all of which are based on magnetic direction.

[[Runway]]s are designated by a number between 01 and 36, which is generally one tenth of the magnetic [[azimuth]] of the runway's [[Course (navigation)|heading]]: a runway numbered 09 points east (90°), runway 18 is south (180°), runway 27 points west (270°) and runway 36 points to the north (360° rather than 0°).<ref>[http://www.faa.gov/air_traffic/publications/ATpubs/AIM/Chap2/aim0203.html Federal Aviation Administration Aeronautical Information Manual, Chapter 2, Section 3 Airport Marking Aids and Signs part 3b] {{webarchive|url=https://web.archive.org/web/20120118165015/http://www.faa.gov/air_traffic/publications/atpubs/aim/Chap2/aim0203.html |date=2012-01-18 }}</ref> However, due to magnetic declination, changes in runway designators have to occur at times to keep their designation in line with the runway's magnetic heading. An exception is made for runways within the [[Northern Domestic Airspace]] of Canada; these are numbered relative to true north because proximity to the magnetic North Pole makes the magnetic declination large and changes in it happen at a high pace.

Radionavigation aids located on the ground, such as [[VHF omnidirectional range|VOR]]s, are also checked and updated to keep them aligned with magnetic north to allow pilots to use their magnetic compasses for accurate and reliable in-plane navigation.

For simplicity aviation sectional charts are drawn using true north so the entire chart need not be rotated as magnetic declination changes. Instead individual printed elements on the chart (such as VOR compass roses) are updated with each revision of the chart to reflect changes in magnetic declination. For an example refer to the sectional chart slightly west of [[Winston-Salem, North Carolina]] in March 2021, magnetic north is 8 degrees west of true north ([http://www.iFlightPlanner.com/AviationCharts/?Map=sectional&GS=115&Route=KINT Note the dashed line marked 8°W]).<ref>[http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/magnetic.html See also CUNY]</ref>

When plotting a course, some small aircraft pilots may plot a trip using true north on a sectional chart (map), then convert the true north bearings to magnetic north for in-plane navigation using the magnetic compass. These bearings are then converted on a pre-flight plan by adding or subtracting the local variation displayed on a sectional chart.

[[global positioning system|GPS]] systems used for aircraft navigation also display directions in terms of magnetic north even though their intrinsic coordinate system is based on true north.  This is accomplished by means of lookup tables inside the GPS which account for magnetic declination. If flying under [[visual flight rules]] it is acceptable to fly with an outdated GPS declination database however if flying [[instrument flight rules|IFR]] the database must be updated every 28 days per FAA regulation.

As a fail-safe even the most advanced airliner will still have a magnetic compass in the cockpit.  When onboard electronics fail, pilots can still rely on paper charts and the ancient and highly reliable device—the magnetic compass.