2.6 Compass Error & Conversions (TVMDC)

Compass Error & Conversions (TVMDC)

Quick Answer: TVMDC is the ladder that converts between True, Magnetic, and Compass directions using Variation and Deviation. Going up the ladder toward True you are "correcting": add East, subtract West. Going down toward Compass you are "uncorrecting": add West, subtract East. You plot courses in true but steer them in compass, so you must be able to travel the ladder in both directions.

This is the single most tested calculation in Navigation General and it reappears in the Chart Plot module. Master the pattern and it becomes automatic.

The Ladder

Write the five steps in order, top to bottom:

The classic mnemonic is "True Virgins Make Dull Company" (or "...Companions"). Variation always sits between True and Magnetic; deviation always sits between Magnetic and Compass. That placement never changes, which is why you must apply variation to get from true to magnetic and deviation to get from magnetic to compass.

The Direction Rule

Which way you add depends on which way you travel the ladder:

• Correcting — going UP (Compass toward True): add East, subtract West. A common tag is "Correcting add East," or the mnemonic "Can Dead Men Vote Twice At Elections" (Compass, Deviation, Magnetic, Variation, True, Add East).
• Uncorrecting — going DOWN (True toward Compass): add West, subtract East. The tail of the first mnemonic, "...Add Whiskey," reminds you to Add West when going down the ladder.

A quick sanity phrase: "East is least, West is best" when correcting means easterly errors reduce the number and westerly errors increase it as you go up toward true. Going the other way, the effect reverses.

Worked Example 1 — Uncorrecting (finding the course to steer)

You have plotted a course on the chart of 095 degrees True. Variation is 14 degrees West and deviation for this heading is 2 degrees East. You want the compass course to steer, so you go down the ladder (True to Compass): add West, subtract East.

1. T to M: apply variation. 095 + 14 (West, add going down) = 109 degrees Magnetic.
2. M to C: apply deviation. 109 - 2 (East, subtract going down) = 107 degrees Compass.

Steer 107 degrees on the compass to make good a 095 true course.

Worked Example 2 — Correcting (finding the true bearing)

You take a bearing of a lighthouse and the compass reads 120 degrees. Deviation for your heading is 5 degrees East and variation is 10 degrees West. You want the true bearing, so you go up the ladder (Compass to True): add East, subtract West.

1. C to M: apply deviation. 120 + 5 (East, add going up) = 125 degrees Magnetic.
2. M to T: apply variation. 125 - 10 (West, subtract going up) = 115 degrees True.

The true bearing is 115 degrees — the value you plot on the chart.

Worked Example 3 — Same-Label Case

Course 200 degrees True, variation 8 degrees East, deviation 1 degree West. Going down (uncorrecting): subtract East, add West.

1. T to M: 200 - 8 (East, subtract going down) = 192 Magnetic.
2. M to C: 192 + 1 (West, add going down) = 193 Compass. Steer 193 degrees.

Notice that even though variation and deviation had opposite labels here, you still apply each one at its own step in sequence rather than netting them first — applying them one at a time, in order, is what keeps you from mixing up which error belongs between which pair.

Checking Your Work

A fast self-check: convert your answer back the other way and confirm you land on the number you started with. In Example 1, take 107 compass and correct it (add East, subtract West): 107 + 2 East = 109 magnetic, 109 - 14 West = 95 true. It matches the 095 you began with, so the conversion is sound. Any time an answer feels off, run this reverse check before committing to a course.

Traps to Avoid

• Applying the wrong error at the wrong step. Variation lives only between T and M; deviation only between M and C. Never apply variation to get from magnetic to compass.
• Forgetting to reverse the sign when you reverse direction. If you add West going down, you must subtract West going up.
• Using the wrong deviation. Deviation depends on the vessel's heading, not on the direction of the object you are sighting, so enter the deviation card with your heading.
• Skipping the magnetic step. When only variation is given (no deviation, or deviation zero), Magnetic and Compass are the same, but write the ladder out anyway to avoid slips.

Because you plot in true but steer in compass, every leg of a passage rides this ladder. Practice both directions until you can convert without hesitating — the exam rewards speed and the water rewards accuracy.