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Related to magnetic variation: Magnetic deviation
magnetic variation[mag′ned·ik ‚ver·ē′ā·shən]
Small changes in the earth's magnetic field in time and space.
In navigation, at a given place and time, the horizontal angle between true north and magnetic north measured in degrees East (+) or West (−) according to whether magnetic north lies east or west of true north. The magnetic variation indicated on a map or a chart is not a fixed quantity but is subject to change. Magnetic variation is indicated by dashed lines on maps and charts. Most maps and charts indicate the rate of annual change of variation also.