FAA Private Pilot Helicopter (PRH)Navigation and AirspaceMedium
On an aeronautical chart, a line connecting points that all have the same amount of magnetic variation is called what, and the specific line where magnetic variation is zero is called what?
- ARhumb line; great circle
- BIsogonic line; agonic line
- CAgonic line; isogonic line
- DCompass rose; isoclinic line
Show answer & explanationAnswer & explanation
Correct answer: B. Isogonic line; agonic line
Isogonic lines connect points of equal magnetic variation, and the agonic line is the specific isogonic line where variation is zero (true north equals magnetic north).
Why the other options are wrong
- A. Rhumb lines and great circles describe navigation paths, not variation.
- C. This reverses the two terms.
- D. A compass rose shows headings on an airport diagram; isoclinic lines relate to magnetic dip, not variation.
Isogonic and Agonic Lines
Isogonic lines on a chart connect points of equal magnetic variation; the agonic line is the isogonic line representing zero variation.
- Variation is the angle between true north and magnetic north
- Isogonic lines are labeled with variation values
- Agonic line = 0° variation
- Variation changes with location, unlike deviation which changes with heading
Memory trick: Agonic = 'A-Gone-ic' — variation is gone (zero).