# Earth's Magnetic Field for Aviation: Variation, Dip, and Directive Force

> Understand Earth's magnetic field components for aviation navigation, including variation, dip, directive force, and compass reliability limits near poles.

- **Subject:** 022 — Instrumentation
- **Section:** 022-03 — Magnetism – Direct Reading Compass and Flux Valve
- **Topic code:** 022-03-01
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/instrumentation/magnetism-direct-reading-compass-and-flux-valve/earths-magnetic-field/

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The Earth acts like a giant magnet with magnetic poles that do not coincide with the geographic poles. This field is fundamental for navigation but introduces specific errors that must be understood.

## Components of the Field
The total magnetic force ($T$) at any point on Earth is resolved into two components:
*   **Horizontal Component ($H$):** Parallel to the Earth's surface. It provides the **directive force** that aligns the compass needle.
    *   Maximum at the **Magnetic Equator**.
    *   Zero at the **Magnetic Poles**.
*   **Vertical Component ($Z$):** Perpendicular to the Earth's surface. It causes the compass needle to "dip" downwards.
    *   Zero at the Magnetic Equator.
    *   Maximum at the Magnetic Poles.

## Magnetic Measurements
*   **Magnetic Variation:** The angular difference between **True North** and **Magnetic North**.
    *   Varies by location.
    *   **Isogonals:** Lines connecting points of equal variation.
    *   **Agonic Line:** A line of zero variation (True and Magnetic North align).
*   **Magnetic Dip (Inclination):** The angle between the horizontal plane and the Earth's magnetic field lines.
    *   Caused by the vertical component ($Z$).
    *   **Isoclinals:** Lines connecting points of equal dip.
    *   **Aclinic Line:** The line of zero dip (Magnetic Equator).

## Limitations
*   The magnetic compass is most effective at the **Magnetic Equator** where the **Directive Force ($H$) is strongest**.
*   Near the magnetic poles, the directive force is too week and the dip is too strong, rendering the compass **unreliable or useless** (sluggish operation and extreme dip).

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.