Isogonals and Agonic Lines

What Are Isogonals?

Isogonals, also called isogonic lines, are curves drawn on the Earth's surface that join points of equal magnetic variation. Magnetic variation (or declination) is the angle between true north (geographic) and magnetic north at a given location. On navigation charts, isogonals are typically marked with their variation value, such as 5°E or 3°W, and are crucial for converting between true and magnetic headings.

The Agonic Line Explained

The agonic line is a unique isogonal where the magnetic variation is exactly zero. Along this line, a compass points directly to true north. The agonic line does not follow a simple path like the equator; instead, it weaves irregularly across the globe, shifting over time as Earth's magnetic field changes. There is only one agonic line, and it can be found in different locations depending on the current magnetic field configuration.

Isogonals and Navigation

Pilots use isogonals to determine the local magnetic variation and apply the correct correction to headings and bearings. Since the Earth's magnetic field is not uniform, isogonals converge toward the magnetic poles and can change over time. Charts often indicate the annual rate of change, so pilots must use up-to-date information to avoid cumulative navigation errors.

Key Differences

• Isogonals: Connect points of equal magnetic variation (not always zero).
• Agonic Line: The specific isogonal where variation is zero.
• Both are essential for accurate navigation, especially when plotting courses over long distances.