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.

Compass Deviation & Variation Correction Tool

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.

Example and tips

With a true course of 090 degrees, a variation of 6 degrees West, and a deviation of 2 degrees East, the magnetic course is 090 − (−6) = 096 and the compass course is 096 − (+2) = 094 degrees to steer. The compass error is −6 + 2 = −4 degrees, that is 4 degrees West. Always read deviation from the card against the compass heading you will actually be steering, since deviation changes as the ship’s head swings.