What does TVMDC stand for?

True, Variation, Magnetic, Deviation, Compass. It is the order of corrections from chart to compass. Going down the list (true to compass) you subtract easterly errors and add westerly; going up you do the reverse.

How do I sign variation and deviation?

By convention East is positive and West is negative. The tool lets you pick the direction so you do not have to remember the sign. Internally it treats east errors as positive degrees and west errors as negative.

What is compass error?

Compass error is the algebraic sum of variation and deviation. It is the single correction between the true heading and the compass heading. East error means the compass reads low; west error means it reads high.

Why does deviation change with heading?

Deviation is caused by the vessel's own magnetic field, which points differently relative to the compass as the ship turns. That is why deviation is read from a card indexed by compass heading, not taken as a single value.

Which way do I apply the corrections?

From true to compass, subtract east and add west. From compass to true, add east and subtract west. The classic memory aid is can dead men vote twice at elections, and this tool applies the rules for you so you only enter values.

What is the Compass Deviation & Variation Correction Tool?

Convert between true, magnetic, and compass headings or bearings by applying chart variation and deviation-card deviation with the TVMDC mnemonic. Built for navigators and sailing students doing chartwork and passage planning. It runs free in your browser on Gera Tools, with nothing uploaded.

Compass Deviation & Variation Correction Tool

Name: Compass Deviation & Variation Correction Tool
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Chartwork lives or dies on getting the sign of variation and deviation right. This tool applies the TVMDC corrections both ways so you can turn a true bearing into a compass course to steer, or work a compass bearing back to true for plotting, without second-guessing whether to add or subtract.

How it works

East errors are treated as positive degrees and West errors as negative. The total compass error is the sum, and the headings relate as follows:

magnetic = true − variation
compass  = magnetic − deviation = true − variation − deviation
compass error = variation + deviation

Going the other way, from compass to true, you add the errors back. Because east is positive, subtracting a positive (east) error matches the rule add east when correcting compass to true and subtract east when going true to compass. The result is wrapped into the 0 to 359 degree range.

Worked example

True course: 090° Variation: 6°W (negative, so −6) Deviation: 2°E (positive, so +2)

Step 1 — True to Magnetic:

magnetic = 090 − (−6) = 096°

Step 2 — Magnetic to Compass:

compass = 096 − (+2) = 094°

Compass error = −6 + 2 = −4°, that is 4° West. To steer 090° true, you point the compass to 094°.

Now working back, checking the reverse: starting from compass 094°, add the 4°W error (positive, because correcting compass to true means adding west error) gives magnetic 096°, then subtract the 6°W variation (add, because west variation corrects upward) gives 090° true. It reconciles perfectly.

Why deviation changes with heading

This is the most common source of chartwork errors. Deviation is caused by the vessel’s own magnetic field — permanent and induced magnetism in the hull, engine, and fittings. Because those objects rotate with the ship, their field aligns differently relative to the compass needle as the ship turns. A deviation of 3°W steering north may become 5°E steering east. This is why the deviation card lists a separate value for each compass heading, and why you must look up the deviation for the compass course you will be steering, not a nearby one.

Practical notes for passage planning

If you are converting a true bearing from the chart into a compass course to steer, go down TVMDC: subtract east, add west at each step.
If you are taking a compass bearing with a hand-bearing compass and want to plot it on the chart as a true bearing, go up TVMDC: add east, subtract west.
The memory aid is Can Dead Men Vote Twice At Elections — each initial matches True, Variation, Magnetic, Deviation, Compass, Add (East to correct up), Error West.
For singlehanded sailing, pre-compute your compass courses before departure and note them on a crib sheet — the calculation is much easier at anchor than at night in a seaway.