Magnetic Variation (Declination): The Angular Difference Between True North and Magnetic North in Navigation

Definition and Core Concept

Magnetic variation (or magnetic declination) is the horizontal angular difference, measured in degrees, between true north—the direction of the geographic North Pole—and magnetic north—the direction indicated by a magnetic compass. This angle is unique for every point on Earth and changes over time due to the constantly shifting nature of the Earth’s magnetic field.

• Easterly variation: Magnetic north is east of true north (variation is positive).
• Westerly variation: Magnetic north is west of true north (variation is negative).

Magnetic variation is critical in navigation for converting between bearings referenced to true north (as seen on most maps and charts) and bearings referenced to magnetic north (as shown on a compass). Navigators must apply variation correctly to avoid substantial errors, especially over long distances or in areas with large variation.

Example:
If the variation is “10° East,” magnetic north is 10° east of true north. To convert a true heading to a magnetic heading, subtract 10°.

Key Terms: True North, Magnetic North, and Navigational Concepts

• True North (Geodetic North): The direction toward the geographic North Pole, used as the universal reference for maps and geodetic measurements.
• Magnetic North: The direction a compass needle points, aligning with the Earth’s magnetic field, which does not coincide with the geographic North Pole and shifts over time.
• Magnetic Heading: Direction indicated by a compass, measured clockwise from magnetic north.
• True Heading: Direction relative to true north, measured clockwise.
• Isogonic Lines: Lines on a map connecting points of equal magnetic variation.
• Agonic Line: The line along which magnetic variation is zero (magnetic north and true north coincide).
• Compass Points: 360-degree graduations on a compass rose, typically referenced to magnetic north.

Why Magnetic Variation Exists: Geophysical Origins

The Earth’s magnetic field is generated by convection currents of molten metal in the outer core—a process called the geodynamo. This field is not perfectly aligned with the rotational axis, leading to a spatial offset between the geographic and magnetic poles.

• The geomagnetic North Pole moves over time due to changes in Earth’s core.
• Local anomalies (e.g., mineral deposits) can also distort the field, causing local variation.

These factors mean the angle between true north and magnetic north (the magnetic variation) is different at every point on Earth and changes gradually (secular variation). To keep navigation accurate, official models like the World Magnetic Model (WMM) are updated every five years.

Magnetic Variation in Navigation

Why It Matters

• Accurate navigation: Ensures courses plotted on charts (true north) match compass headings (magnetic north).
• Safety: Prevents navigational drift and errors, which can be hazardous in aviation and maritime operations.
• Runway numbering: Runways are labeled based on magnetic alignment, requiring periodic updates as variation changes.

Practical Application

• On Charts: Most maps use true north as reference; compasses use magnetic north.
• Conversion Required: Navigators convert between true and magnetic headings using the local variation.

Finding and Applying Magnetic Variation

How to Find Local Variation

• Aeronautical/nautical charts: Show isogonic lines labeled with variation and direction.
• Official databases: Use NOAA Declination Calculator .
• GPS and apps: Many modern devices automatically provide variation data.

How to Apply Variation

Standard Formulas

Example:

• Seattle, WA: Variation is 16° east.
• True course: 270° (due west).
• Magnetic heading = 270° – 16° = 254°.

A small miscalculation can cause significant drift, especially over long distances.

Real-World Examples

Aviation

A pilot plans a flight from Boston (variation 15° west) on a true heading of 270°:

• Magnetic heading = 270° (true) + 15° (west) = 285° (magnetic).

Maritime

A ship leaves San Francisco (variation 14° east) on a true course of 000°:

• Magnetic heading = 000° – 14° = 346°.

Land Navigation

A hiker in Denver (variation 9° east) wants a bearing of 120° true:

• Magnetic heading = 120° – 9° = 111°.

Runway Renumbering

If a runway originally numbered 18/36 (180°/360° magnetic) shifts to 175°/355° due to variation, it must be renumbered (e.g., 17/35) to match the new magnetic heading.

Isogonic Lines, Agonic Line, and Compass Points

• Isogonic Lines: Marked on charts; show locations of equal variation.
• Agonic Line: Where variation is zero; magnetic and true north coincide.
• Compass Points: 360-degree reference, essential for navigation; always clarify if reference is true or magnetic.

Historical and Scientific Context

• Discovery: First recognized by Chinese navigators; corrected for by European mariners in the Age of Exploration.
• Science: Magnetic field generated by the geodynamo; not aligned with rotation axis, leading to variation.
• Modeling: World Magnetic Model and International Geomagnetic Reference Field provide updated variation data for navigation.

Changes Over Time (Secular Variation)

• Secular Variation: Magnetic variation changes over years and decades as the magnetic poles drift.
• Impacts: Runway numbers, navigation charts, and compass corrections must be updated regularly.
• Forecasting: Models like WMM are recalibrated every five years using satellite and observatory data.

Modern Technology and Magnetic Variation

• GPS Integration: GPS and avionics incorporate current variation automatically.
• Database Updates: Navigation systems regularly update variation data.
• Redundancy: Navigators are still trained to apply variation manually in case of electronic failure.
• Emerging Tools: Augmented reality and advanced navigation apps now visualize and apply variation in real time.

Common Errors and Tips

• Neglecting variation: Leading to course errors.
• Sign confusion: Misapplying east/west variation.
• Outdated data: Using old charts or variation values.
• Local anomalies: Ignoring local magnetic disturbances.

Tip:
Always check the latest variation for your location, verify chart dates, and practice conversion formulas regularly.

Magnetic variation is a foundational element of safe, accurate navigation. With the Earth’s magnetic field in constant flux, staying informed and vigilant about variation is essential for pilots, mariners, surveyors, and anyone relying on a compass.