1. True, Magnetic & Compass Direction

True direction is what is shown on maps — the direction to the geographic North Pole. An INS (Inertial Navigation System) finds True direction by detecting Earth's rotation. Most aircraft, however, use magnetic compasses as primary heading references, with INS as a standby or supplement.

The Earth behaves as though a huge, slightly bent bar magnet runs through it, with its magnetic poles approximately (but not exactly) aligned with the True poles. This misalignment creates the need to convert between True, Magnetic, and Compass headings in flight planning.

2. Variation — Definition & Observer Geometry

• A line joining points of equal variation is an Isogonal (shown as a pecked/dashed line on charts).
• The isogonal of zero variation is the Agonic Line.

How Observer Position Determines Variation

The variation at any point depends on the geometry of the observer, the True North Pole, and the Magnetic North Pole:

• Observer A: Magnetic North Pole is to the right of True North → Easterly variation
• Observer B: Magnetic North Pole is to the left of True North → Westerly variation
• Observer C/D: Further from both poles → smaller angle of variation
• Observer E: On the Great Circle joining True and Magnetic Poles → zero variation (on the Agonic Line)

3. Situation at the Poles — Maximum Variation

For an observer on the arc of the Great Circle between the True North Pole and the Magnetic North Pole:

• True North points one way (toward the True Pole)
• The compass needle points in exactly the opposite direction (toward the Magnetic Pole)
• The variation at such a point is exactly 180°

4. The Real Variation Map — Isogonals & Agonic Lines

The real magnetic field is more complex than the idealised model — it resembles a bent bar magnet. The North Magnetic Pole (NERC 2000 survey) was at approximately 81°N 110°W; the South Magnetic Pole at approximately 63°S 135°E — they are not antipodal.

There are two Agonic Lines (zero-variation lines), both starting from the True North Pole:

• One runs southward through Western Europe → Stuttgart, Germany → Central Africa → back via North-Central Asia → Australia → South Magnetic Pole
• The other runs southward through the USA → South America → True South Pole

5. Changes in Variation Over Time

Variation at any point changes with time because the Magnetic Poles are moving. There are several overlapping cycles:

6. Updating Isogonals on Charts

For frequently republished radio-navigation charts, isogonals are usually current enough. For topographical maps republished every 5–10 years, pilots may need to update isogonals during flight planning. Charts show the year of origin and the annual change in one of two ways:

7. Magnetic Dip — Angle, H and Z Components

The Earth's magnetic field acts along a total force vector T, which is directed into the Earth at an angle to the horizontal. This angle is the Angle of Dip.

Resolving T into H and Z

Relationship Between Dip and H

• At the magnetic equator: Dip = 0°, H ≈ T (maximum — compass most effective)
• As latitude increases toward the poles: Dip increases, H decreases, Z increases
• At the magnetic poles: Dip = 90°, H ≈ 0 (compass unreliable)
• At a typical mid-latitude UK location: Dip ≈ 66°

Magnetic field strength is measured in microteslas (µT). H is the directive force. A line joining points of equal magnetic dip is an Isoclinal. The isoclinal of zero dip is the Aclinic Line (the magnetic equator).

8. The 6 µT Threshold & Compass Reliability

The accepted threshold below which H is too weak to reliably drive a magnetic compass is 6 microteslas (µT). Inside the 6 µT zone, compass reliability cannot be guaranteed.

9. Deviation — Definition & Sign Convention

The aircraft itself contains magnetic influences (metal structures, electrical currents, equipment) which deflect the compass needle from its ideal alignment with Magnetic North. The compass then points to Compass North — not Magnetic North.

Sign Convention (±)

Applied to Compass heading to give Magnetic heading: Hdg(M) = Hdg(C) + Deviation (using algebraic sign).
Equivalently: From Compass to Magnetic, the signs are true — East is +, West is −.

10. Applying Variation & Deviation — CADIZ Rules

Worked Examples — Variation

Worked Examples — Deviation

Full T → M → C Example

11. All Definitions