Which type of line indicates that zero variation exists and no correction is needed?

The agonic line is pivotal in navigation as it indicates where the magnetic north and true north align, resulting in zero magnetic variation. This means that at any point along the agonic line, navigators do not need to make any adjustments between their magnetic compass readings and true headings.

This feature is crucial for pilots and navigators since it simplifies navigation processes in regions where the agonic line passes through, as it eliminates the need for corrections that are typically required in areas where magnetic variation exists. Consequently, the agonic line serves as a reliable reference point, enhancing accuracy in navigation without the added complexity of variance.

In contrast, the other types of lines mentioned—isogonic, true, and magnetic—are linked with different aspects of navigation and require varying degrees of correction based on the magnetic variation present in specific geographical areas.