isogonic

Isogonic describes lines or curves connecting points of equal angles, most commonly appearing in maps showing magnetic declination—the angle between true north and magnetic north.

An isogonic line connects all points where a compass needle deflects the same number of degrees from true north.

The pilot consulted the isogonic lines on the aeronautical chart to adjust for compass variation.

In geometry, isogonic can also describe figures or transformations preserving angular relationships.