# TVMDC Explained

by Ian Fortey Updated on January 16, 2021. In

*Boat Safe is a community supported site. We may earn comission from links on this page, but we have confidence in all recommended products.*

TVMDC is a kind of acronym known as a mnemonic initialism. It’s an easy to remember abbreviation word. That means each letter in the acronym stands for the first word in a longer term. It stands for true heading, variation, magnetic heading, deviation, compass heading. Sometimes it will be called true variation magnetic deviation compass. Some people remember it by saying TV Makes Dull Children. So what does it mean? TVMDC is used for correcting courses when you’re out on a boat.

When trying to navigate at sea, you need to understand each of true variation magnetic deviation compass. This will allow you to correct your heading so you’re going where you need to go. You can calculate all five values, even if you only have three of them handy. Let’s take a look at what each one is and a handy way to calculate the navigation formula.

To start calculating your TVMDC you should have the numbers arranged like so:

**True** – This number is always positive and between 0 and 360.

**Variation** – This number can be positive or negative. If it’s Westward, then it will be positive if it’s Eastward, then it will be negative.

**Magnetic** – This number is always positive and between 0 and 360.

**Deviation** – This number can be positive or negative. If it’s Westward, then it will be positive, if it’s Eastward, then it will be negative.

**Compass** – This number is always positive in between 0 and 360.

When doing the math for your TVMDC, you will add the numbers up going down the scale. So True plus Variation plus Magnetic and so on. If you’re going up, then you’ll be subtracting. So Compass minus Deviation minus Magnetic and so on.

## Calculations

Let’s take a look at a navigation formula to see how it works. We’ll start with a True course of 50°. Our Variation is going to be 2° east. And the Deviation can be 20° west.

True: 50°

Variation: – 2

Magnetic:

Deviation: + 20

Compass:

We’re just going to use simple math to fill in the blank spots here. In this case start with 50 and then add -2. Which means take away two. So our magnetic is going to be 48.

True: 50°

Variation: – 2

Magnetic: 48°

Deviation: + 20

Compass:

Once we have established that, we can find the Compass by adding the 20 for the Deviation. So the Compass is going to be 68.

True: 50°

Variation: – 2

Magnetic: 48°

Deviation: + 20

Compass: 68°

What this means is if you want to get a True course of 50°, then you’re going to need to adjust your Compass heading to 68°. Again, if you have any three of these figures, you can calculate the other two simply. You could also do them in reverse from Compass up to True by subtracting rather than adding your way down.

## What These Terms Mean

Calculating the values for TVMDC is one thing. But it’s worth knowing what each of the terms means in order to understand why and how you’re calculating it. The numbers used to calculate TVMDC come from the way the globe is segmented. This is based on the Geographic North Pole and the Magnetic North Pole. Because the Earth is a sphere, it’s divided into 360 degrees. The directions are listed as degrees based on that, relating to magnetic fields.

**T**: This is the direction that you are intending to travel.

**V:** This is the difference between Magnetic North and Geographic North

**M:** This is the direction that the compass is indicating is north

**D:** The difference between magnetic north and the direction the compass says is North. Local magnetic interference is why there is a difference here.

**C:** This is what you need to set your compass to in order to reach your True Direction. You can do this after accounting for Variation and Deviation.

## The Bottom Line

Navigating by compass is not widely done these days. Modern technology has eliminated the need for many people. However, when all else fails, TVMDC will not steer you wrong. If you’re serious about sailing, you should know how to do these calculations. If you can master them, you’ll always know where you are and where you’re going.

Categories: Navigation