Definition
Iron-rich rock, ore bodies and volcanic formations can distort the local field by several degrees or occasionally tens of degrees over limited areas. Charts annotate such areas as Local Magnetic Anomaly, sometimes with the observed extent, for example in parts of the Baltic, around Iceland and off western Australia. Since the effect changes with position rather than heading, it cannot be corrected by adjusting the compass, and navigators should cross check with GNSS course or transits when passing through.
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