Editing Magnetic declination (section)

=== Floating magnetic card compasses {{anchor|Magnetic bearing}} ===
[[Image:Compensating magnetic declination.svg|300px|thumb|left|How to compensate for magnetic declination when reading a compass. In this example, the declination is 14°E (+14°), so the compass card points to a "north" 14 degrees to the East of true North. To obtain a true bearing, add 14 degrees to the bearing shown by the compass.]]

Compasses that utilize a floating magnetized dial or card are commonly found in marine compasses and in certain models used for land navigation that feature a [[Lensatic compas|lensatic]] or [[prismatic]] sighting system.  A floating card compass always gives bearings in relation to magnetic north and cannot be adjusted for declination.  True north must be computed by adding or subtracting local magnetic declination.  The example on the left demonstrates a typical conversion of a magnetic bearing from a floating card compass to a true bearing by ''adding'' the magnetic declination.  The declination in the example is 14°E (+14°).  If, instead, the declination was 14°W (−14°), you would still "add" it to the magnetic bearing to obtain the true bearing:  40°+ (−14°) = 26°.

Conversely, local declination is ''subtracted'' from a true bearing to obtain a magnetic bearing.  With a local declination of 14°E, a true bearing (i.e. obtained from a map) of 54° is converted to a magnetic bearing (for use in the field) by subtracting declination:  54° – 14° = 40°.  If the local declination was 14°W (−14°), it is again subtracted from the true bearing to obtain a magnetic bearing:  54°- (−14°) = 68°.
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