A compass deviation card is an essential navigation tool created after “swinging” your boat’s compass to measure and record compass errors caused by onboard magnetic interference. This card helps you correct compass readings for safe and precise steering.

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⛵ 1. What Is a Compass Deviation Card?

The deviation card lists the compass error — or deviation — for different headings, typically recorded every 30 degrees from 000° to 360°. Because deviation depends on your vessel’s magnetic environment and varies with heading, this card is unique to each boat.

Unlike magnetic variation, which is a geographic factor consistent for all vessels in a region, deviation arises from ferrous metals and electronic devices onboard and changes as the boat turns.

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🌊 2. How Is a Deviation Card Created? (Swinging the Compass)

To produce an accurate deviation card, the compass must be “swung” — a process best done annually:

• Locate charted transits (fixed landmarks aligned on the chart).

• Steer your vessel along these transits, aligning your heading precisely with the transit line.

• Account for magnetic variation (local geographic correction) to find the magnetic heading.

• Compare the vessel’s compass reading to the magnetic heading derived from the transit.

• The difference between compass heading and magnetic heading is the compass deviation for that particular heading.

• Record this deviation on the card.

• Repeat this process for transits every 30 degrees until the card is complete.

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📍 3. Using Your Deviation Card in Navigation

Once you have a current deviation card, date it and keep it at your navigation station for reference. It will help you convert bearings between True, Magnetic, and Compass courses accurately.

When converting courses, use the mnemonic:

• True Virgins Make Dull Company (T V M D C):

    • Start with the True course (from the chart)

    • Apply Variation (geographic) to get Magnetic course

    • Apply Deviation (from your card) to get Compass course

• Or in reverse, use CADET to remember correction directions (add East, deduct West).

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🚀 4. Why Is This Important?

Correct application of your compass deviation card ensures that the course you steer matches the intended course on your chart. This reduces navigational errors, keeps your vessel safe, and improves overall confidence in your nautical skills.