How did Edmond Halley’s magnetic charts show compass declination?

Edmond Halley’s magnetic charts, published in 1701, represented compass declination—the angle between magnetic north and true north—by using curved lines drawn across the Atlantic Ocean and parts of the Pacific Ocean.1

These curved lines, known as isogonic lines, connected geographical positions in the oceans that shared the same compass declination. Halley compiled these charts using both his own measurements, gathered during his 1698–1700 scientific voyage aboard the Paramore Pink, and all other available observations at the time. While intended to assist with navigation and potentially help determine longitude at sea, the charts were not widely adopted for that purpose because compass declination is difficult to measure with high accuracy and is subject to change over time.1