Variation Diagram

A variation diagram is the note printed inside a chart’s compass rose stating the magnetic variation (the angle between true north and magnetic north) at that location, its year of observation, and the annual rate of change. The navigator reads off the variation, applies the annual change to the chart’s year to bring it to date, then applies it to convert between true and magnetic bearings. Variation differs by place and drifts over time as Earth’s magnetic field shifts; isogonic lines on small-scale charts show its distribution.