Compass Variation and Deviation: T-V-M-D-C

TL;DR — Variation is the angular difference between true north and magnetic north for a given geographic location; deviation is the error introduced by the vessel's own magnetic field acting on the compass needle. Every person directing a towing vessel must know and apply both corrections, and every regulated vessel must carry a current deviation table in the wheelhouse.

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What the Rule Says

Equipment requirements

33 CFR §164.35 establishes the baseline equipment standard for vessels subject to that subpart. It requires:

• An illuminated magnetic steering compass, mounted in a binnacle, readable at the vessel's main steering stand. 33 CFR §164.35
• A current magnetic compass deviation table or graph, or a compass comparison record for the steering compass, kept in the wheelhouse. 33 CFR §164.35

For commercial fishing vessels, 46 CFR §28.230 independently requires each vessel to be equipped with an operable magnetic steering compass and a compass deviation table at the operating station. 46 CFR §28.230

For small passenger vessels (subchapter T), 46 CFR §184.402 requires a suitable magnetic compass designed for marine use, mounted at the primary operating station. Exceptions exist for vessels on a rivers route, non-self-propelled vessels, and vessels operating on short restricted routes on lakes, bays, and sounds. On any vessel not limited to daytime operations, the compass must be illuminated. 46 CFR §184.402

Operational requirement to apply corrections

33 CFR §164.78 goes beyond equipment and imposes an operational duty on towing vessels. The person directing and controlling the movement of the vessel must know and apply the variation and deviation, where a magnetic compass is fitted and where charts or maps have enough detail to enable this type of correction. 33 CFR §164.78

This is not a suggestion — it is a mandatory performance standard for the watch officer.

Where variation is found on a chart

The compass rose printed on a nautical chart shows both a true (outer) ring and a magnetic (inner) ring, with the local variation and annual change noted at the centre of the rose. Light List — Magnetic / Compass Rose

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The T-V-M-D-C Framework

The standard mnemonic used to convert among true, magnetic, and compass headings or bearings is:

T — V — M — D — C True — Variation — Magnetic — Deviation — Compass

Working from True toward Compass (i.e., converting a true course to a compass course to steer), you apply variation first, then deviation. Working from Compass toward True (i.e., correcting a compass reading to obtain a true bearing), you reverse the sequence.

A second mnemonic helps with sign convention:

"Correcting, add East" (or equivalently, "Can Dead Men Vote Twice? At Elections")

When correcting (going from Compass → True), add easterly errors and subtract westerly errors. When uncorrecting (going from True → Compass), do the opposite: subtract easterly errors and add westerly errors.

Definitions

Variation is the angle between the geographic (true) meridian and the magnetic meridian at a given location. It is caused by the non-coincidence of the geographic and magnetic poles. Variation is a property of the Earth's magnetic field at a specific place and changes slowly over time. The annual rate of change is printed on the chart compass rose alongside the current value. Light List — Magnetic / Compass Rose

Deviation is the angular difference between magnetic north and the direction the compass needle actually points, caused by the vessel's own magnetic influences — ferrous metal in the hull, engines, electrical wiring, and installed equipment. Deviation changes with the vessel's heading and must be determined by compass adjustment or by swinging the vessel and recording the error on each heading. The result is recorded in the deviation table that must be kept current and available in the wheelhouse. 33 CFR §164.35 46 CFR §28.230

Compass error is the algebraic sum of variation and deviation. It represents the total angular difference between true north and the direction the compass card reads.

Worked example — True to Compass

Suppose the chart shows a true course of 085°T. The compass rose shows variation of 12°W. The deviation table for the vessel on that heading shows 3°E.

Step 1: Apply variation (uncorrecting, so reverse the rule — subtract East, add West): 085°T + 12°W = 097°M

Step 2: Apply deviation (uncorrecting — subtract East): 097°M − 3°E = 094°C

The helmsman steers 094° by compass to make good 085° true.

Worked example — Compass to True

A compass bearing of 215°C is observed. Deviation on that heading is 4°W. Variation is 10°E.

Step 1: Apply deviation (correcting — add East, subtract West): 215°C − 4°W = 211°M

Step 2: Apply variation (correcting — add East): 211°M + 10°E = 221°T

The true bearing of the object is 221°T.

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Why It Matters on the Exam

OUPV and Master 100 GT written exams consistently test this topic in two ways:

1. Calculation problems. You will be given a true course or compass bearing and asked to convert it, or vice versa. The numbers will include a mix of easterly and westerly errors to test whether you apply the sign convention correctly. Errors of 10–20 degrees are common in the distractors, corresponding exactly to what you would get if you added instead of subtracted (or vice versa).

2. Regulatory knowledge questions. You may be asked where variation is found (the chart compass rose), what document must be kept in the wheelhouse (the deviation table or compass comparison record), or which regulation requires the operator of a towing vessel to apply both corrections. 33 CFR §164.35 33 CFR §164.78 Light List — Magnetic / Compass Rose

The exam also tests the distinction between the two errors: variation is a geographic property found on the chart; deviation is a vessel-specific property found in the deviation table aboard the vessel. Confusing the two is the single most common error candidates make.

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Common Pitfalls

Pitfall 1 — Reversing the direction of travel through T-V-M-D-C. Candidates who memorize the mnemonic but not the direction of application frequently apply corrections in the wrong order or apply the wrong sign. Always establish first whether you are going True → Compass (uncorrecting) or Compass → True (correcting), then apply the rule consistently.

Pitfall 2 — Treating variation as vessel-specific. Variation is determined by geographic location and is read from the chart compass rose. Light List — Magnetic / Compass Rose It does not change when the vessel changes heading. Deviation, by contrast, is heading-dependent and is read from the vessel's deviation table. 33 CFR §164.35 Mixing these up produces wrong answers on both calculation and regulatory questions.

Pitfall 3 — Ignoring annual change. The compass rose states both the current variation value and the annual rate of change. If the chart is several years old, the candidate must apply the annual change to obtain the current variation. Exam problems sometimes provide a chart date and a current year specifically to test this step.

Pitfall 4 — Assuming deviation is constant. Deviation varies with the vessel's heading. The deviation table lists values for compass headings at intervals (commonly every 15° or 30°). For a heading between tabulated values, interpolation is required. Using the wrong column of the table — or using a single deviation value for all headings — is a frequent calculation error.

Pitfall 5 — Confusing compass error with deviation. Compass error = variation + deviation (algebraically). Some exam questions ask for compass error; others ask for deviation alone. Read the question carefully.

Pitfall 6 — Regulatory confusion about who must carry what. The deviation table requirement applies under 33 CFR §164.35 (vessels subject to that subpart) and independently under 46 CFR §28.230 (commercial fishing vessels). The operational duty to apply both corrections is stated in 33 CFR §164.78 for towing vessels. 33 CFR §164.35 46 CFR §28.230 33 CFR §164.78 Candidates sometimes believe the deviation table is optional or that only the compass itself is required.

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Quick Check

Q1 — Where does a mariner find the local magnetic variation for a given chart area?

Variation is found on the compass rose printed on the nautical chart. The rose shows both a true (outer) ring and a magnetic (inner) ring, with the local variation value and annual rate of change noted at the centre. Light List — Magnetic / Compass Rose

Q2 — What document must be kept in the wheelhouse to account for compass deviation, and under what regulation?

A current magnetic compass deviation table or graph, or a compass comparison record for the steering compass, must be kept in the wheelhouse. 33 CFR §164.35 For commercial fishing vessels, a compass deviation table must be at the operating station. 46 CFR §28.230

Q3 — True course is 270°T. Variation is 8°E, deviation is 2°W. What is the compass course?

Going True → Compass (uncorrecting): subtract East variation, add West deviation. 270° − 8°E = 262°M 262° + 2°W = 264°C The compass course to steer is 264°C.

Q4 — A compass bearing reads 340°C. Deviation is 5°E, variation is 15°W. What is the true bearing?

Going Compass → True (correcting): add East deviation, subtract West variation. 340° + 5°E = 345°M 345° − 15°W = 330°T The true bearing is 330°T.

Q5 — Under 33 CFR §164.78, what is the operational duty of the person directing a towing vessel with respect to compass errors?

The person directing and controlling the movement of the vessel must know and apply the variation and deviation, where a magnetic compass is fitted and where charts or maps have enough detail to enable this type of correction. 33 CFR §164.78

Q6 — Which vessels are exempt from the compass requirement under 46 CFR §184.402?

Vessels on a rivers route, non-self-propelled vessels, and vessels operating on short restricted routes on lakes, bays, and sounds need not be fitted with a compass. 46 CFR §184.402

Q7 — What is the difference between variation and deviation?

Variation is the angular difference between the geographic (true) meridian and the magnetic meridian at a specific geographic location; it is a property of the Earth's field and is read from the chart compass rose. Deviation is the angular error caused by the vessel's own magnetic influences acting on the compass needle; it is heading-dependent and is recorded in the vessel's deviation table. Light List — Magnetic / Compass Rose 33 CFR §164.35