A line that represents equal magnetic variation is referred to as?

The term being referred to in the question is "isogonic line." Isogonic lines are lines drawn on a map to indicate areas of equal magnetic variation or declination, which is the angle difference between magnetic north and true north at any given location. Understanding isogonic lines is essential for navigation, as pilots and navigators need to account for this variation when using compasses, which point to magnetic north rather than true north.

This concept is particularly important because the Earth's magnetic field is not uniform, and variations can change based on geographic location. Isogonic lines help in identifying how much the compass reading differs from the true heading, allowing for precise navigation.

Other options do not accurately represent this concept. A "true line" typically refers to a line representing true direction or position, while an "agonic line" specifically denotes the line where magnetic variation is zero, meaning there's no difference between magnetic north and true north at that point. The term "magnetic line" is not a standard term in this context and does not specifically indicate the concept of equal magnetic variation.