True, Variation, Magnetic, Deviation, Compass: The TVMDC Conversion, Properly Explained

True, Variation, Magnetic, Deviation, Compass — TVMDC — is the standard sequence for converting a true course or bearing down to what you'd actually read on a magnetic compass, and back again. The mechanics are simple once the two sources of error are clearly separated.

Two Different Errors, Two Different Causes

Variation is the angle between true north and magnetic north at your position — caused by the Earth's own magnetic field, published on the chart, and changing slowly with location and year. Deviation is different: it's caused by the ship's own magnetism, changes with the ship's heading, and comes from the deviation card specific to that vessel and that compass position.

Worked Example: True to Compass

Say your true course is 090°. Variation at your position is 4°W. Applying variation to true gives magnetic: 090° + 4° (west variation is added when going true to magnetic) = 094° magnetic. Now apply deviation for that heading from the deviation card — say 2°E — subtracted going magnetic to compass: 094° - 2° = 092° compass.

Worked Example: Compass Back to True

Going the other direction reverses every sign. From 092° compass, add back the 2°E deviation to get magnetic (094°), then subtract the 4°W variation to get back to true (090°). This is exactly where most people make mistakes under exam pressure — the direction of the sign flips depending on which way you're converting.