Magnetic Variation vs Deviation: What Every Navigator Must Know

A magnetic compass does not point to true north. Two separate errors, variation and deviation, pull the compass needle away from the direction shown on a nautical chart. One is caused by the Earth. The other is caused by the vessel. Confusing the two, or failing to correct for either, leads to course errors that compound over distance.

For any navigator working with a magnetic compass, understanding both errors is fundamental to safe passage planning.

What Is Magnetic Variation?

Magnetic variation is the angular difference between true north and magnetic north at any given location on Earth. True north is the geographic North Pole. Magnetic north is the point toward which a compass needle aligns, located in the Canadian Arctic and constantly shifting due to changes in the Earth's molten core.

Because the two poles do not coincide, a compass reading at any position will differ from the true bearing by a certain number of degrees. That difference is magnetic variation, expressed in degrees east or west of true north.

Variation changes depending on geographic position. Along agonic lines, variation is zero. On either side of these lines, variation increases. Variation also changes slowly over time as the magnetic poles shift, typically by a few minutes of arc per year.

What Is Magnetic Deviation?

Magnetic deviation is the error caused by magnetic influences aboard the vessel itself. Steel hulls, engines, cargo, electrical wiring, and electronic equipment all generate magnetic fields that interfere with the compass needle, adding a second layer of error on top of variation.

Unlike variation, deviation is unique to each vessel and changes with heading. A compass that reads accurately on a north heading may show several degrees of error when the vessel turns to the east or south. Even two compasses installed on the same vessel at different locations can show different deviation values.

Deviation is expressed in degrees east or west. Easterly deviation means the compass needle is deflected east of magnetic north. Westerly deviation means the compass is pulled west.

Key Differences Between Magnetic Variation and Magnetic Deviation

Variation and deviation are both compass errors, but they originate from different sources. Variation is caused by the Earth's magnetic field and affects all compasses equally at the same geographic position. Deviation is caused by the vessel's magnetic environment and is unique to each ship, each compass, and each heading.

Variation can be predicted and is published on nautical charts . Deviation must be measured aboard the specific vessel through compass swinging . Variation changes slowly over the years. Deviation can change when cargo is loaded, structural modifications are made, or new electrical equipment is installed near the compass.

How to Convert Between True, Magnetic, and Compass Bearings

The relationship between the three norths follows a simple formula. True bearing equals compass bearing plus or minus deviation, plus or minus variation. Stated as a sequence: Compass plus deviation equals Magnetic, and Magnetic plus variation equals True.

The CADET Rule

A widely used memory aid is CADET: Compass, ADd East for True. When converting from compass bearing to true bearing, add easterly errors and subtract westerly errors. When converting from true bearing to compass bearing, reverse the process: subtract easterly errors and add westerly errors.

For example, assume a vessel's compass reads 090 degrees. Deviation on that heading (from the deviation card) is 3 degrees east, and the variation for the area (from the chart's compass rose ) is 5 degrees west. Following CADET: 090 + 3 (east deviation) = 093 magnetic. 093 - 5 (west variation) = 088 true.

Getting this conversion wrong by even a few degrees means missing a waypoint by several nautical miles on a long ocean passage.

Where to Find Variation on a Nautical Chart

Every nautical chart includes at least one compass rose printed on the chart face. The compass rose has two concentric rings. The outer ring is aligned to true north. The inner ring is aligned to magnetic north. The angular difference between the two rings is the variation for that location.

Printed in the center of the compass rose is the variation value and its annual rate of change. A typical notation reads: "Var 4° 30' W (2020), decreasing 8' annually." To apply this information in 2026, a navigator calculates six years of change: 6 x 8 minutes = 48 minutes. Updated variation: 4 degrees 30 minutes west minus 48 minutes = 3 degrees 42 minutes west.

Keeping chart-derived variation current is a basic part of passage planning . Maintaining properly corrected nautical charts aboard ensures accuracy throughout the voyage.

How Deviation Is Measured and Recorded

Deviation is determined through a procedure called compass swinging, where the vessel is slowly rotated through all cardinal and intercardinal headings (N, NE, E, SE, S, SW, W, NW) while the compass reading on each heading is compared against a known reference, typically the gyrocompass or a transit bearing aligned to a charted range.

The difference between the compass reading and the known magnetic heading on each heading is the deviation for that heading. After the swing, a qualified compass adjuster uses corrector magnets and soft iron spheres mounted on the binnacle to reduce the deviation as much as possible. Residual deviation that cannot be eliminated is recorded on a deviation card, which is posted near the compass for the watch officer's reference.

According to ISO 25862:2019, magnetic compasses must be adjusted at least every two years. SOLAS Chapter V, Regulation 19 requires all vessels (excluding fishing vessels and pleasure craft under 150 GT) to carry a properly compensated magnetic compass with a current deviation card. ANS offers professional magnetic compass adjustment services for commercial vessels and yachts, including swing-ship operations, deviation curve reporting, and adjustment certificates.

When Compass Adjustment Is Required

Beyond the standard two-year interval, compass adjustment should be performed after structural repairs near the compass, installation or removal of electrical equipment on the bridge, loading of magnetically significant cargo (such as steel coils or iron ore), long periods docked on the same heading, and deperming operations.

Port state control officers check for a current deviation card during inspections. A missing or expired card can result in a deficiency. Maintaining proper SOLAS documentation aboard supports compliance readiness.

Why Variation and Deviation Still Matter on Modern Vessels

Modern bridge navigation equipment relies on gyrocompasses and GPS, neither of which is affected by magnetic variation or deviation. However, the magnetic compass remains mandatory under SOLAS because it operates without electrical power.

During a total power failure or gyrocompass malfunction, the magnetic compass becomes the only heading reference available. An officer who cannot apply variation and deviation corrections cannot safely navigate. Regular practice comparing the magnetic compass against the gyrocompass and recording discrepancies in the compass deviation book helps maintain proficiency and flags developing problems early.

Get Your Compass Right

A well-adjusted magnetic compass is not just a regulatory checkbox. On the day the electronics go dark, a properly corrected compass and a navigator who understands variation and deviation are the most critical safety assets aboard. For compass adjustment services, deviation cards, or navigation instruments, contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.

Frequently Asked Questions

Q. What is the difference between magnetic variation and deviation?

Magnetic variation is the angular difference between true north and magnetic north, caused by the Earth's magnetic field. Magnetic deviation is the difference between magnetic north and the direction a ship's compass actually points, caused by magnetic influences aboard the vessel.

Q. How do you find magnetic variation for a location?

Magnetic variation is printed on the compass rose of every nautical chart, along with the annual rate of change. Navigators calculate current variation by applying the annual change from the date printed on the compass rose to the current year.

Q. What is a compass deviation card?

A deviation card is a document that records the residual compass error on each heading after a compass adjuster has minimized deviation. The card is posted near the compass and used by the watch officer to correct compass readings on any heading.

Q. How often should a ship's compass be adjusted?

ISO 25862:2019 requires compass adjustment at least every two years. Adjustment is also necessary after structural modifications, changes to bridge equipment, or loading of magnetically significant cargo.

Q. Does deviation change when the ship changes heading?

Yes. Deviation varies with the vessel's heading because the relative position of the ship's magnetic influences changes in relation to the compass as the vessel turns. A compass may show zero deviation on one heading and several degrees on another.

Q. Do modern ships still need magnetic compasses?

Yes. SOLAS Chapter V, Regulation 19 requires all vessels to carry a magnetic compass or equivalent heading reference that operates independently of any power supply. The magnetic compass serves as the primary backup when gyrocompass or GPS systems fail.