Which term refers to lines indicating constant magnetic declination?

• A. Isogonic
• B. Isopleths
• C. Vanishing Point
• D. Fiducial Mark

Answer: (A)

Lines of equal magnetic declination are called isogonic lines. They show where the angle between true north and magnetic north is the same, so navigators can apply a single correction to compass readings to get true bearings across that region. On a map, each isogonic line is labeled with the declination value (east or west) and these lines bend with the geography, converging toward the poles. This concept specifically identifies lines of constant magnetic variation, making them distinct from general contour-like lines (isopleths) that apply to any measured value, and from unrelated terms like vanishing point (perspective drawing) or fiducial marks (reference marks on a map).