isogonic

Isogonic is an adjective derived from the Greek words iso meaning equal and gonia meaning angle. In geomagnetism and navigation, the term describes lines on the Earth’s surface that connect locations where the magnetic declination—the angle between true north and magnetic north—has the same value. These lines are known as isogonic lines and are used on navigational charts to help adjust compass readings to true bearings. The declination value is typically expressed in degrees east or west of true north.

Isogonic lines are not static; the Earth's magnetic field changes over time, causing the lines to drift gradually. Modern isogonic charts are produced from geomagnetic field models, such as the International Geomagnetic Reference Field, and are updated to reflect secular variation and short-term fluctuations. Navigators rely on isogonic information to translate magnetic compass data into accurate geographic directions.

The concept also intersects with the idea of isogonic points, which are locations where the magnetic declination is zero; the line of zero declination is commonly referred to as the agonic line. While the broader term isogonic can appear in other scientific contexts to describe equal-angle relationships, its most widely used meaning remains tied to lines of constant magnetic variation in geomagnetism and navigation.