Compass Variation and Deviation — Correcting and Uncorrecting with T-V-M-D-C

What the rule requires

Compass error is the angular difference between a compass direction and its corresponding true direction, and it has two parts that must both be applied: variation and deviation Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶1. They come from different places, they change for different reasons, and every exam question on this topic turns on keeping them straight.

Variation belongs to the place

Variation is the angular difference between true and magnetic north, and it varies according to geographic location. You read it from the inside of the compass rose on the chart, labelled easterly or westerly. It remains the same for all headings of the vessel.

Variation is easterly when the northerly part of the magnetic meridian lies to the right of the true meridian, westerly when it lies to the left Bowditch Pub. 9 Ch. 8 §829.

The rose also carries an annual increase or decrease. To bring it up to date, subtract the year printed at the centre of the rose from the present year, multiply that difference by the annual change, and apply the result to the charted value. Predictions are good for only a few years, so use the latest chart available .

Deviation belongs to the vessel, on that heading

Deviation is the deflection influenced by the vessel and its electronics on the compass, and it varies according to the heading. Metal objects around the compass, electrical motors and the boat itself all produce it. It is not affected by the vessel's geographic location Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶2.

Because it changes with heading, deviation cannot be a single number. It is determined by "swinging ship" — running a charted range, taking multiple observations while pivoting about a piling, or running several ranges — and tabulated for every 15° of the compass. The posted table lists compass course, deviation and magnetic course side by side, so the deviation for a heading can be drawn straight out of it Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶3.

Two administrative facts get tested. The original table goes in the permanent boat record with a copy posted near the compass, and a new table is required annually, after yard availabilities, and after any addition or deletion of equipment or structural alteration affecting the magnetic characteristics of the boat.

Do not tolerate deviations of more than 3°. Beyond that the compass is adjusted — northerly heading first, removing half the observed error on the N/S compensating magnet with a nonmagnetic tool, then southerly, then easterly and westerly on the E/W magnet, repeating until the residual is minimal — or handed to a professional compass adjuster Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶4.

You run the same 090° heading out of a Chesapeake berth and then, three weeks later, out of a berth in Maine. Which error has changed?

Variation. Its value changes with geographic location and is read from the rose nearest your area of operation. Deviation on 090° is unchanged, because deviation follows the heading and the vessel's own magnetism, not where the vessel happens to be.

The two ladders

Variation always sits between true and magnetic; deviation always sits between magnetic and compass Bowditch Ch. 6 §602. Which way you climb decides the signs.

Correcting is going from compass toward true — C, D, M, V, T — and you add easterly errors and subtract westerly. The standard device is Can Dead Men Vote Twice At Election, where the tail carries the sign: Add Easterly .

Uncorrecting is going from true toward compass — T, V, M, D, C — and you subtract easterly errors and add westerly. The paired device is True Virtue Makes Dull Company After Wedding: Add Westerly.

The add-rule belongs to the direction you are travelling, not to the error. The same 5°E deviation is added on the way up to true and subtracted on the way down to the compass.

The separate memory aid compass least, error east; compass best, error west does something different: it labels an error you have just observed. When the compass reading is less than the magnetic, the error is east; when the compass reading is greater, the error is west . It does not tell you which way to apply that error afterwards.

Telling it apart

The criterion is what makes the value change.

Variation — changes with position, never with heading. Source: the inner scale of the chart's compass rose Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶1.
Deviation — changes with heading, never with position. Source: the vessel's own deviation table. Most often misfiled when a bearing is involved: the deviation for a bearing comes from the heading being steered, not from the bearing you read Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶5.
Gyro error — changes with neither. A gyrocompass is not influenced by magnetism, so it is not subject to variation or deviation, and its error is constant and equal around the horizon Bowditch Pub. 9 Ch. 8 §821. An error of 1°E applies to every bearing taken by it Bowditch Pub. 9 Ch. 8 §820.

For boat work you will be in magnetic almost all the time, since true bearings come from a gyrocompass and boats do not normally carry one .

Working a question

Case 1 — a course off the chart to an order for the helm

Parallel rulers give a true course of 221°. The rose shows variation 9°E. The deviation table shows 2°W. Find the compass course Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶4.

Decide the direction of travel first. You have a charted direction and you need something to steer, so you are going from true toward compass — uncorrecting, T V M D C, add westerly.
Write the column, one letter per line, before touching any arithmetic: T = 221°, V = 9° E, M = ?, D = 2° W, C = ?. The column is what keeps you from applying variation where deviation belongs.
Variation acts between T and M. Uncorrecting subtracts easterly errors, so 221° − 9° = M = 212°.
Deviation acts between M and C, and the posted table is set up to give deviation against magnetic course, so 212° is the row you enter with.
Uncorrecting adds westerly errors, so 212° + 2° = C = 214°. That is the number you give the helm.

Had you climbed the other ladder by habit and added the 9°E, you would have ordered 232° and put the vessel 18° off track.

Case 2 — a bearing you intend to plot

Your vessel is on a heading of 263° M. You sight Kays Pt. Light across the compass and read 060°. The deviation table, entered on the heading of 263°, gives 7°W Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶5.

A compass bearing must be corrected before it can be plotted. Correcting from C toward M subtracts the westerly error: 060° − 7° = 053° M. With the bearing in magnetic it goes against the inner ring of the rose, the same ring your courses are read from.

The decision that carries the question is step one: you entered the table with 263°, the heading being steered, not with 060°. A bearing is not a course.

Compass course 127°, variation 4°W from the rose, deviation 5°E from the table. What is the true course?

128°. You are correcting, so C D M V T and add easterly: 127° + 5°E = 132° M, then subtract the westerly variation, 132° − 4° = 128° T.

Where candidates lose the point

Adding east in both directions. The candidate who has only ever heard "add east" runs the 221° / 9°E / 2°W problem to 232° and finds it among the options. Uncorrecting subtracts easterly and adds westerly; the mnemonic you invoke has to match the direction you are travelling.
Entering the deviation table with the bearing. With a bearing of 060° taken from a heading of 263° M, the 060° row is on the sheet in front of you and looks like the obvious entry. Deviation always depends on the vessel's heading Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶5.
Swapping the dependencies. Answers that call for a new deviation table after shifting operating areas, or a fresh variation after a course change, both invert the rule: variation remains the same for all headings, and deviation is not affected by geographic location Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶2.
Treating a charted range direction as magnetic. Where a man-made range has its direction printed on the chart, that direction is in degrees true, not magnetic. Take it as magnetic while swinging ship and every deviation in the table is wrong by the local variation.
The reciprocal. Reading the wrong side of the rose gives a bearing 180° out — 092° in place of 272°. Picture the vessel at the centre of the rose looking out toward the object.
Reading "compass best, error west" as an instruction. It names the error's direction from a comparison; the add-or-subtract decision comes from the ladder you are on.
Correcting a gyro course. Variation and deviation have no application to a gyrocompass, which is not influenced by magnetism Bowditch Pub. 9 Ch. 8 §821.

Check yourself

True course 350°, variation 6°W, deviation 3°E. What compass course do you steer?

353°. Uncorrecting, T V M D C, add westerly: 350° + 6°W = 356° M; then subtract the easterly deviation, 356° − 3° = 353° C.

You are steady on compass heading 000°. Crossing a charted range whose magnetic bearing measures 272°, you read the range across the compass as 266°. What is the deviation on this heading?

6°E. The compass reading is less than the magnetic, so compass least, error east. The magnitude is the 6° difference, and it applies to a heading of 000° only.

Your deviation curve shows 5°W on several headings. What does that call for?

Adjustment. Deviations of more than 3° are not tolerated; adjust the compass with the compensating magnets, removing half the observed error on each of the four cardinal headings in turn, or employ a professional compass adjuster Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Magnetic Compass ¶3.

The compass rose nearest your operating area shows 8°W variation, annual increase 6', dated 2019. It is now 2024. What variation do you use?

8°30'W. Five years elapsed, at 6' of increase per year, is 30' to be applied to the charted 8°W. Variation is a property of position, so this figure holds for every heading you steer in that area.

Your vessel has just come out of a yard period in which a steel radar mast and a new electric windlass were fitted. The posted deviation table is nine months old. Is it good for another three months?

No. A new table is required after yard availabilities and after any addition of equipment or structural alteration that would affect the magnetic characteristics of the boat, independent of the annual requirement.

You take a compass bearing of a tank while steering 105° C, on which heading the table shows 0° deviation. Can you plot the bearing as read?

Yes, as a magnetic bearing. With zero deviation the compass bearing and the magnetic bearing are the same value, and a magnetic bearing plots against the inner ring of the rose. Any non-zero deviation on the heading steered would have to be applied first, since a compass bearing must be corrected before it can be plotted.

The gyro error is 1°E. Does it change when you come from 010° round to 250°?

No. Gyro error is independent of the Earth's magnetic field and is constant in one direction, so 1°E applies to all bearings around the compass. That is the opposite behaviour to deviation, which changes with every heading.