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Magnetic variation is the angular difference between true north and magnetic north, which is the direction the local magnetic field points. This direction changes from place to place on earth and slowly over time. The compass needle aligns with the direction of the magnetic field, but that direction is not pointed to the magnetic pole. The field direction varies notably around the earth. At any high latitude, magnetic north can vary many degrees in both directions as you travel toward the pole. See maps of this direction in the link below.

Variation is marked on all nautical charts along with its the slow rate of change, rarely more them 6' or 7' per year, except for very high latitudes. On paper charts, variation is marked East or West and the change is marked increasing or decreasing, but on electronic navigational charts (ENC) only algebraic signs are used. East is + and West is . Thus when the variation and the change have the same sign, the value is increasing with time; when they are opposite, the variation is decreasing.

Read more about it worldwide along with a calculator for specific values at https://www.ngdc.noaa.gov/geomag/declination.shtml. On land this is called magnetic declination, but in marine navigation we reserved "declination" for the latitude of stars on a star globe. See also extended notes on variation in ENC.

magnetic anomalies of several or even more degrees over a local region are not uncommon and are usually charted. They are caused by iron ore deposits or locations of lightning strikes... or man made influences such as overhead or under water power lines, rail tracks, tanks, and so on.

An overview of global variation can be seen at https://www.ngdc.noaa.gov/geomag/WMM/data/WMM2020/WMM2020_D_BoZ_MILL.pdf

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