Britannica Sources used in this answer

Why can regional geomagnetic anomalies affect compass readings?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

Regional geomagnetic anomalies affect compass readings because they represent deviations from the Earth's primary, large-scale dipolar magnetic field. While a compass is designed to align with the Earth's main north-south magnetic field, these anomalies—often caused by variations in subsurface geological structures and the composition of igneous and metamorphic rocks—create localized magnetic influences.123

When a compass needle encounters these regional anomalies, it experiences a deflection known as deviation. Because the Earth's total magnetic field is a superposition of the main dipole field and these non-dipole, regional components, the needle does not point solely toward the magnetic pole but is instead pulled by the local magnetic intensity. Consequently, in areas where these anomalies are significant, such as in certain polar regions, the compass may provide inaccurate directional readings unless corrected for these specific local magnetic variations.451