Magnetic Bearing: The Compass Standard in Navigation

What Is a Magnetic Bearing?

A magnetic bearing is the angular direction to a specific point, measured clockwise from magnetic north. When you hold a compass and determine the angle between where the needle points (magnetic north) and a destination, you’re measuring its magnetic bearing. This is the most practical bearing for navigators in aviation, marine, and land environments because it aligns with what a magnetic compass displays in the real world.

For example, if you want to walk from point A to point B and your compass shows the direction to B as 120° from magnetic north, the magnetic bearing is 120°. This is the angle you’d follow to reach your destination, using the compass as your tool.

Magnetic bearings are at the heart of field navigation:

• Aviators use them for headings, approaches, and runway alignment.
• Mariners use them to steer courses and plot directions relative to buoys and landmarks.
• Hikers and land navigators use them to traverse terrain where visible paths are absent.

Because the Earth’s magnetic field changes over time, the difference between magnetic north and true north (called variation or magnetic declination) must always be accounted for to maintain accuracy.

The Role of Magnetic Bearing in Navigation

Magnetic bearing is the actionable reference for virtually all practical navigation using a magnetic compass. Here’s how it’s used:

1. Plotting a Course

Navigators determine the direction from their current position to a destination as a magnetic bearing. This may be found directly with a compass or by converting a true bearing (from a map/chart) using the local magnetic variation.

2. Fulfilling Headings

Pilots fly magnetic headings because aircraft compasses and directional gyros are aligned with the Earth’s magnetic field. Runway numbers, for instance, are based on their magnetic orientation, rounded to the nearest 10°.

3. Marine Navigation

Ships are steered using magnetic bearings shown on the ship’s compass. Courses between waypoints, buoys, or land features are plotted and maintained using these bearings, with correction for deviation as needed.

4. Land Navigation

Hikers set their compass to a magnetic bearing and follow it, allowing them to cross landscapes without visible trails. Orienteers use magnetic bearings to find checkpoints or features on their maps.

5. Triangulation

By taking magnetic bearings to multiple known features, navigators can determine their exact position on a map—an essential skill in both marine and land navigation.

The “Norths” in Navigation

Understanding magnetic bearing requires distinguishing between various reference “norths”:

• True North: The direction to the geographic North Pole, used in mapping and plotting true bearings.
• Magnetic North: The direction a compass needle points, determined by Earth’s magnetic field. Magnetic north drifts over time and differs from true north by an angle called variation.
• Grid North: The north along the grid lines of a map projection (e.g., UTM), useful for mapwork but often slightly offset from true north due to projection distortion.
• Compass North: The direction shown by a compass in its current environment, which may differ from magnetic north due to local magnetic deviation.

Applying Corrections

To translate between these references:

• True to Magnetic: Apply the current variation (subtract east, add west).
• Magnetic to Compass: Apply the local deviation from the compass’s deviation card.
• Compass to True: Correct for both deviation and variation.

Variation (Magnetic Declination) and Deviation

Variation (Declination)

The angular difference between true north and magnetic north at a given location. Variation changes with both place and time due to the movement of the magnetic poles. It’s always marked on maps and charts (e.g., “VAR 7°E, decreasing 8’ annually”).

Deviation

The error in compass readings caused by local magnetic influences (e.g., steel structures, electronics, or even cargo). Deviation is unique to each vessel or aircraft and is charted on a deviation card. Both variation and deviation must be corrected for accurate navigation.

Calculating and Using Magnetic Bearings

From Compass to Map

1. Read the compass bearing (the raw angle).
2. Correct for deviation (if known).
3. Correct for variation to obtain the true bearing for plotting on a map.

From Map to Compass

1. Measure the true bearing on the map.
2. Apply local variation to find the magnetic bearing.
3. Apply deviation to find the compass bearing to steer.

Mnemonic:

• “East is least, West is best” (subtract east variation, add west variation).

Real-World Examples

Aviation

• Runway Numbers: Runway 09 means a magnetic bearing of 090°. If magnetic variation changes over time, runway numbers may be updated.
• VOR Radials: VOR navigation aids provide radials (bearings) relative to magnetic north so pilots can easily set their instruments and fly the correct heading.

Marine

• Plotting a Course: A navigator uses the ship’s compass to steer a magnetic bearing, correcting for deviation using the deviation card and for variation as needed.
• Taking Sightings: Mariners use magnetic bearings to plot fixes or avoid hazards.

Land Navigation

• Orienteering: Competitors use compasses to set and follow magnetic bearings between checkpoints.
• Hiking: Backpackers use magnetic bearings to traverse open terrain or dense forests, translating map bearings to compass bearings with variation correction.

Staying Up to Date: Why Magnetic Bearing Changes

Earth’s magnetic field is in constant flux. The north magnetic pole moves tens of kilometers per year, which means variation (the difference between true and magnetic north) also changes. Navigational charts, aviation databases, and marine compasses must be updated regularly with the latest World Magnetic Model (WMM) or similar reference.

If variation is not updated, courses, runway alignments, and navigation aid radials may become inaccurate, potentially leading to navigational errors.

Magnetic Bearing vs. Other Bearings

• Magnetic Bearing: Angle from magnetic north (real-world compass direction).
• True Bearing: Angle from true north (map/chart reference).
• Compass Bearing: What you read directly on your compass, including deviation.
• Grid Bearing: Angle from grid north (map grid lines, used in military/surveying).

Each has its place—knowing which reference you are using is essential for accurate navigation.

Key Takeaways

• A magnetic bearing gives the direction to a destination, measured clockwise from magnetic north.
• It is the standard for compass-based navigation in aviation, marine, and land travel.
• Navigators must correct for variation (between true and magnetic north) and for deviation (local magnetic influences).
• Regular updates to magnetic variation data are crucial for accuracy.
• Understanding and applying magnetic bearings ensures safe, efficient, and reliable navigation, even in the age of GPS.

Further Reading & Resources

• NOAA World Magnetic Model
• FAA Aeronautical Information Manual: Navigation Aids
• Royal Institute of Navigation: Bearings Explained
• US Geological Survey: Magnetic Declination

Magnetic bearing remains a cornerstone of practical navigation—simple, reliable, and essential for navigating the world, from the open ocean to remote mountain trails.

Frequently Asked Questions

What is the difference between magnetic bearing and true bearing?

A magnetic bearing references magnetic north (compass needle), while a true bearing references geographic north (the North Pole). The difference between them is called variation or magnetic declination.

Why does magnetic bearing change over time or location?

Because the Earth’s magnetic field—and thus magnetic north—moves, the angle between magnetic and true north (variation) changes with both location and time, affecting magnetic bearings.

How do I convert a true bearing to a magnetic bearing?

Subtract the local east variation (or add west variation) from the true bearing. For example, if the true bearing is 090° and the variation is 10°W, the magnetic bearing is 100°.

What is deviation and how does it affect magnetic bearing?

Deviation is compass error caused by local magnetic fields (like metal or electronics near the compass). It must be corrected to ensure the compass accurately indicates the intended magnetic bearing.

Are magnetic bearings used in GPS navigation?

Most GPS units use true north by default, but many can display bearings relative to magnetic north for compatibility with compass-based navigation.

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