What function does an isogonic line serve on a VFR Navigation Chart?

An isogonic line on a VFR Navigation Chart represents areas where the magnetic variation is equal. This means it connects geographic points that share the same magnetic declination, which is the angle difference between true north and magnetic north. Pilots rely on this information to adjust their heading for accurate navigation, particularly since magnetic variation can vary significantly depending on the region and can affect the performance of compasses. By providing a visual reference of where these variations occur, isogonic lines help pilots maintain their course and improve overall navigation accuracy.

The other functions listed—connecting points of equal elevation, marking regulated airspace, and showing the location of navigational aids—pertain to different elements of the chart and serve distinct purposes unrelated to magnetic variation.