An isogonic line joins areas of equal what?

The answer is rooted in the fundamental concepts of navigation and geomagnetism. An isogonic line specifically represents areas on a map where the magnetic declination is the same. Magnetic declination is the angle between magnetic north and true north, which varies depending on geographical location due to the Earth's magnetic field fluctuations.

Understanding that the term "isogonic" directly relates to magnetic declination helps clarify the significance of this term in aviation and navigation. Pilots and navigators rely on magnetic declination to adjust their compasses to ensure accurate headings. Therefore, isogonic lines are crucial for creating charts that facilitate this adjustment and enhance navigational accuracy.

The other options, while related to important concepts in navigation, do not fit the specific definition of an isogonic line. Magnetic variation, for instance, is often used interchangeably with magnetic declination in some contexts, but it more generally refers to the difference across larger areas rather than the specific lines of equal declination. Pressure is relevant to weather patterns in aviation but does not relate to isogonic lines. Elevation, which pertains to altitude above mean sea level, is another separate metric unrelated to the magnetic compass and navigation.