2.4 Magnetic Variation and Navigation

In the old days (before GPS), the magnetic compass was the main instrument used to navigate by pilotage.

Unlike a GPS, the magnetic compass does not point to geographical North (True North), but to Magnetic North. Pilots therefore had to translate headings from magnetic to true, and vice versa. You still need to do so because it’s examinable.

Note

A compass is a poor instrument for a hang glider pilot because a compass is only accurate in steady, level flight.

When an aircraft turns, the compass will lag for a northerly turn and lead for a southerly turn.

When an aircraft accelerates going east-to-west or west-to-east, the compass indicates a northerly turn. When the aircraft decelerates, the compass indicates a southerly turn.

Acceleration and deceleration have no effect on a north-south heading.

In any case, a hang glider is not an aircraft that spends much time in steady level flight.

1. The Earth’s Magnetic Field

The Earth is a magnet with a north and south magnetic pole. Lines of force flow between these poles, creating a magnetic field that surrounds the Earth.

A compass needle is influenced by the Earth’s magnetic field and will lie parallel to one of these magnetic lines of force.

The magnetic poles are not co-located with the geographical poles (True North). Therefore, the north direction indicated by a compass is different from geographical north.

In the image above, TN represents True North and MN represents Magnetic North. The red lines are magnetic meridians. From these lines one can deternime the effect of local magnetic variations on a magnetic compass.

2. Magnetic Variation & Chart Lines

The angle between the direction indicated by the compass and the true geographical direction is called magnetic variation. This magnetic variation is indicated on VNCs so users can apply the required corrections.

Isogonic Lines

Isogonic lines are lines joining points that have the same magnetic variation.

Check the VNC legend to see what isogonic lines look like (dashed curved magenta lines, roughly north to south).

The variation for a specific isogonic line is shown on the VNC along the line (for example, 14°W). You can see this in the VNC excerpt from section 2.3.

Agonic Line

The isogonic line with zero magnetic variation (the only line where True North is the same as Magnetic North) is called the Agonic line.

Did you know?

The magnetic poles are not stationary. The north magnetic pole rotates around the geographical North Pole in an east-to-west circle, completing that circle roughly every 960 years.

As a result, magnetic variation changes continuously. If you compare the same VNC several years apart, magnetic variation may have changed significantly.

3. Converting Headings

If the magnetic pole lies west of the geographical pole from a given point, the compass needle points west of True North, so the variation is westerly.

If the compass needle points east of True North, the variation is easterly.

To convert magnetic headings to true headings, subtract westerly variation or add easterly variation. To convert true headings to magnetic headings, do the opposite.

You can use the mnemonic “East is Least, West is Best” to remember which operation to perform.

West Variation

• True Heading = Magnetic Heading - West variation
• Magnetic Heading = True Heading + West variation

East Variation

• True Heading = Magnetic Heading + East variation
• Magnetic Heading = True Heading - East variation

Example: A magnetic heading of 135° near the 13°W isogonic line means a true heading of 135° - 13° = 122°.

Magnetic Variation Practice

Use this interactive tool to practice converting between true headings and magnetic headings.

Magnetic Variation Tool

Conversion

True to MagneticMagnetic to True

Magnetic variation

WestEast

14°W

True Heading (TH)

°

Enter 0 to 359 degrees.

Result

Magnetic Heading (MH): 104°

MH = TH + Variation

MH = 90° + 14° = 104°

• Magnetic North (14°W)
• True Heading (90°)
• Magnetic Heading (104°)

Flight note: A compass is most reliable in steady, level flight. Turns and acceleration can cause errors.