Which term refers to lines with equal magnetic declination?

Which term refers to lines with equal magnetic declination?

• A. Isogonic
• B. Moire
• C. Isopleths
• D. Fiducial Mark

Answer: (A)

Isogonic lines connect points where the magnetic declination—the angle between true north and magnetic north—is the same. Since this angle varies across the map, these lines show where a compass would need the same adjustment to point toward true north, helping navigators correct their bearings. The zero-declination case is a related idea called the agonic line, but the general term for lines of equal declination is isogonic. Other options refer to unrelated concepts: moire is an optical interference pattern, isopleths are generic lines of equal value, and fiducial marks are reference points on a map.

Isogonic lines connect points where the magnetic declination—the angle between true north and magnetic north—is the same. Since this angle varies across the map, these lines show where a compass would need the same adjustment to point toward true north, helping navigators correct their bearings. The zero-declination case is a related idea called the agonic line, but the general term for lines of equal declination is isogonic. Other options refer to unrelated concepts: moire is an optical interference pattern, isopleths are generic lines of equal value, and fiducial marks are reference points on a map.