Earth Magnetism — Page 62, Lesson 72

Let’s start with the core definitions, because everything in navigation direction builds on them. Heading is the direction in which the fore and aft axis of the aircraft is pointing. That’s the nose-to-tail line. It may be measured from True North, Magnetic North, or Compass North — so you’ll hear “true heading,” “magnetic heading,” or “compass heading.” Variation is the angle measured at a point between the True North direction and the direction indicated by a freely suspended compass needle influenced only by the Earth’s magnetic field. So it’s the angular difference between True North and Magnetic North at your location. It’s termed East or West according to whether Magnetic North lies to the East or West of True North. Deviation is the angle measured at a point between the direction indicated by a compass needle and the direction of Magnetic North. So it’s the error caused by the aircraft’s own magnetic influences — the difference between what the compass shows and actual Magnetic North. It’s termed East or West according to whether Compass North lies to the East or West of Magnetic North. Now the lines on charts. Isogonals are pecked lines on a map or chart joining places of equal magnetic variation. So along one isogonal, variation is the same everywhere. Agonic Line is the name given to isogonals joining places of zero variation. So it’s the specific isogonal where variation is zero. The Angle of Dip is the angle in the vertical plane between the horizontal and the Earth’s magnetic field at a point. So it’s how much the field tilts downward from horizontal — steep near the poles, shallow near the equator. Isoclinals are lines on a map or chart joining places of equal magnetic dip. Aclinic Lines is the name given to isoclinals joining places of zero dip. And note this important point: Isoclinals and Aclinic lines do not appear on navigation charts. So you’ll see isogonals and the agonic line on charts, but not the dip lines. Now, the last part of the excerpt is a bit fragmented — it seems to be part of a multiple-choice question about the magnetic equator or the agonic line. It mentions that something “is the shorter distance between the respective True and Magnetic North and South poles” and that it “follows separate paths out of the North polar regions, one currently running through Western Europe and the other through the USA.” That’s describing the agonic line — the line of zero variation — which indeed runs through Western Europe and through the USA, and it’s the shorter distance between the True and Magnetic poles. So to tie it together: variation is the angular difference between True and Magnetic North, deviation is the compass error from the aircraft, isogonals show equal variation, the agonic line is zero variation, dip is the vertical tilt of the field, isoclinals show equal dip, and aclinic lines are zero dip — and those dip lines aren’t on navigation charts.