The two terms sound similar because both affect magnetic-compass work. Their sources are different. One belongs to the place and date; the other belongs to the compass installation and heading.

Keep a heading's reference attached to the number. Writing only “090°” leaves open whether it is true, magnetic, or compass. Add T, M, or C in training work and in any calculation where confusion is possible. The small label protects the meaning while you move through the correction chain.

The two errors come from different sources, so they are recorded differently and updated on different triggers.

Dimension Variation Deviation
Source Earth's magnetic field Magnetic influences aboard the boat — metal, wiring, engines, electronics
Connects True north to magnetic north Magnetic reading to what the installed compass shows
Recorded on The chart's compass rose The boat's deviation card, from a compass swing
Changes with heading No — changes by location and time Yes — can differ as the vessel turns
When to update Per the chart's stated annual change After equipment changes near the compass, or inconsistent readings

What is variation?

Charts are referenced to true north, the geographic direction toward the pole. A magnetic compass aligns toward magnetic north. The angle between them is variation, also called magnetic declination.

Variation changes by location and slowly over time. A chart's compass rose may print a value, the year for that value, and an annual change. Update it only as the chart instructs. A value copied from another area or old chart may not fit your route.

Do not apply the annual change blindly beyond the chart's intended use. Use current official chart information and follow its notes. Electronic chart systems may display orientation and magnetic information differently, so know which reference the system is showing before comparing it with a steering compass.

Harbour and boats representing a place-specific magnetic direction

What is deviation?

Metal, wiring, engines, speakers, electronics, and other magnetic influences aboard can deflect the steering compass. That boat-specific error is deviation. Its size and direction may differ as the vessel turns because the relationship between the compass and onboard magnetism changes.

A compass swing compares known headings and produces a deviation card. The card belongs to that compass in that installation. Moving equipment near the compass can make the old information unreliable.

Deviation is not necessarily one fixed correction for every direction. Read the row or curve for the relevant heading and interpolate only according to the card's method. If the compass or nearby equipment changes, treat the old card cautiously and arrange an appropriate check rather than assuming the error stayed the same.

Boat rigging and a docked sailboat representing onboard influences

Apply the Corrections

Think in three references: true, magnetic, and compass. Variation connects true with magnetic. Deviation connects magnetic with what the installed compass reads. Work through both in the correct direction for the task.

Do not memorise a sign rule without understanding the labels. Write the starting reference and required result, then take variation from the current chart and deviation from the vessel's card. Keep east or west signs exactly as the source states.

Check the result by reversing the calculation. Start from the answer and work back through the same two corrections to recover the original direction. If it does not return, inspect the signs, source values, and reference labels. This catches a transcription error without requiring a different memory phrase.

Sailboat underway while a navigator applies compass corrections

Check the Compass

Inspect the compass for bubbles, damage, poor lighting, or an unreadable lubber line. Keep portable electronics and magnetic objects away. Confirm that a current deviation card is available and identify headings with larger corrections.

Compare compass behaviour with suitable independent references as part of normal navigation practice. If equipment has changed or readings are inconsistent, arrange a competent check. A GPS display does not repair an unreliable magnetic compass.

Record the date and circumstances of a check. A comparison made while the boat is moving, near large metal structures, or with temporary equipment beside the compass may be misleading. Use suitable references and competent practice. The purpose is to understand the installed compass, not force it to agree with one convenient display.

Docked sailboat where the steering compass can be checked

Label every number before calculating

Write T, M, or C beside each course or bearing: true, magnetic, or compass. Variation connects true and magnetic references. Deviation connects magnetic and compass references. Do not apply a correction until both the starting and required reference are named.

Take variation from the current chart or official source for the place and date. Take deviation from the boat's current deviation information for the relevant heading. These corrections come from different causes and sources, so one value cannot stand in for the other.

Work in both directions

Practise converting a true course from the chart into the compass course the helm will steer. Then reverse the exercise from a compass observation back to a true reference for plotting. Keep the signs and intermediate magnetic value visible rather than doing the chain mentally.

The compass steering guide shows how the helm holds the resulting number without S-curves. A correct conversion still needs good steering and a check of the actual track.

Keep deviation boat-specific

Deviation comes from magnetic influence around the installed compass and can vary with heading. Check the deviation card rather than copying a value from another boat. Keep portable electronics, tools, speakers, and other magnetic objects away from the compass during use and checks.

If equipment changes, the compass develops a bubble or damage, or comparisons become inconsistent, arrange a competent check. A GPS display can provide an independent navigation reference in suitable conditions, but it does not repair the magnetic compass or remove the need to understand its error.

Avoid false precision

Use values only to the precision supported by the chart, card, compass, steering, and task. A long decimal does not make uncertain source data more accurate. Record the chart edition or source, date, heading, and circumstances so another navigator can audit the result.

Current, leeway, steering error, and wind can still move the boat away from the intended ground track after the compass conversion is correct. Compare position and track with suitable independent information and revise the navigation plan.

Run a four-line check

For each exercise, write starting reference, variation source, deviation source, and final reference. Have another person check the labels before the arithmetic. If the labels are wrong, a neat answer is still wrong.

On the boat, locate the compass, lubber line, deviation card, and objects that could disturb the reading. Connect the paper chain to the installed equipment. That final step turns a memory rule into a navigation process the crew can explain and repeat.

SailStarter's compass-error lesson lets you practise the chain without guessing. On paper, draw three lines labelled true, magnetic, and compass. Place variation between the first two and deviation between the last two. Then use one current training chart and a sample deviation card to work in both directions, writing every reference beside each number. Finish aboard by locating the actual compass, lubber line, and deviation card. The habit of naming the reference at every step prevents more mistakes than a clever rhyme.

Make three practice cards with different headings and deviation values. For each, state the starting reference, required reference, source of variation, and source of deviation before doing arithmetic. Have another person check the labels first and the numbers second. Navigation becomes easier to audit when the reasoning is visible on the page.

Work one example with no correction needed before adding east or west values. This confirms that the reference chain is understood independently of sign rules. Then add a charted variation and one deviation-card entry, writing the intermediate magnetic course.

Check the result in reverse. If the forward and reverse chains do not return to the starting value, inspect labels and signs before repeating arithmetic. This simple round trip catches an error that a plausible final heading can hide.

Keep course conversion separate from position fixing. A correctly converted compass course does not prove the boat is on the intended track. Compare with charted position, depth, visual references, and other suitable navigation information.

When passing a course to the helm, state "compass" with the number. When recording a chart course, state "true." If magnetic is used as an intermediate or operational reference, name it. Clear speech preserves the work done on paper.

Finally, inspect the deviation card's date and heading coverage. If the card is missing, unreadable, or inconsistent with observed behaviour, arrange a competent check rather than assuming zero deviation. Unknown is a condition to resolve, not a convenient number.

Record each conversion and its source in the sailing logbook so another navigator can audit it.