TVMDC – Navigator’s Calculator


Download TVMDC from iTunes…

Check out TVMDC on YouTube

TVMDC is a simple to use calculator App for Sailors, Boaters and Navigators – who are learning to Navigate with Charts

Learning to Navigate a Boat?

A important skill for any and all Sailors. And a simple skill to pick up, once you follow some basic steps.

So let’s do it. First things first. Have a good ol’ stare at your Chart. Open up a Navigation chart in a new tab so that you can refer to it. You can view and download free chart on the NOAA web site here

An example chart of San Francisco bay

You’ll notice big round compasses at perhaps a couple of places on your chart. These are called your Compass Roses. There are more than one, as the magnetic Variation may differ, even across your chart. Sometimes there are three or four. The magnetic Variation is the difference in the value of Magnetic North to Compass North, at the position of each Compass Rose. It’s the difference between the points around which the earth spins, the North & South Poles, and the point that your magnetic compass will point to, the Magnetic North direction. They are not the same and they even move apart each year! You can see that the two circles are not aligned to each other. They are rotated, or misaligned by an offset. This offset is the magnetic Variance. It might be small or large or VERY large. Depending on where you are on the planet. This change is because the planet is made up of magnetic Stuff that channels magnetic flux to a greater or lesser extent, depending upon how much Iron, Nickel makes up the planet at that location.

It’s an important difference if you are learning to Navigate. Understanding this Rose is the first step in learning to navigate the seas…


Inside each Compass Rose are some numbers that describe this difference. The Variance. You can see the value of Variance, in the Rose above is: 4 degrees and 15 minutes, and it was measured in 1985.  The Variance is not fixed in time however, so you need to note both the value of the variance in 1985 and the annual change since then. It changes as the Magnetic Poles move each year. So, you also need to note the date that the Chart was published. With these three numbers: Variance (in degrees) , the direction of the Variance: West of East, the Annual Change (in minutes) (which could be an Annual Increase or a Annual Decrease) and Year of Publication, (shown as 1985, in the above graphic) you’re now ready to input these values into your TVMDC App.

The final bit of information your TVMDC App needs to know is the error associated with your ship’s Compass. This has nothing to do with your Chart but has all to do with your boat. Think of this as a Calibration Error. It’s caused by any ferros (Iron, Nickel, Cobalt and some others) metal in your boat. If you boat was made of plastic & wood and Aluminum only, and does not have an Iron engine, this calibration error is probably close to zero. This error is calculated by the guy who installs your compass. It’s done once for your boat, when your engine is installed and is a fixed error. If you don’t know it, leave it as zero. If you do know it, it may be written on your ship’s compass itself. It’s called the Ship’s Deviation, or more specifically the Ship’s Compass’s Deviation.

Input into TVMDC, your Navigation Chart’s Magnetic Variation, Annual Variation Increase and the Date of the Rose. – shown on your Chart’s Compass Rose, along with your ship’s Compass Deviation.

You’re done!.. for the TVMDC screen, at least.

TVMDC is now able to quickly move between Magnetic, True and Compass bearings.

Now type in bearings of any type: True, Compass or Magnetic, and the correction to the other two types of bearing will be shown for you.

You now have a handy calculator to instantly switch from Compass to True, or from True to Compass.

Remember, True bearings are those referred to on your Chart. Charts lines, parallel and perpendicular to the Latt. and Long grid are all drawn as True. But, underway, while stearing your boat, you would use Compass Bearings. So it’s important to be able to move back and forth between these two bearing types.

The difference can be big! … depending upon where you are. Very big if you’re Very North or Very South….

True – Variation – Magnetic – Deviation – Compass

Next up! TVMDC also calculates Course to Steer. A term used by Sailors describing the corrected Course to Steer so as to account for Set, Drift and Leeway.

Set & Drift have to do with winds and currents that drift your boat in a direct other than the one you’re steering in. Drift is the speed of this Drift and Set is the Direction of your Drift.

Imagine trying to sail across a big river. The River’s Drift will push the boat down stream, perhaps perpendicular to the way you’re trying to sail. So you have to steer upstream a bit to account for the flow of the river.

Your TVMDC App firstly calculates your Set and Drift vectors and then uses them to give you the best direction to steer, accounting for the drift. To do this, a user inputs a heading and two fixes. TVMDC calculates the Set & Drift subjected to the craft.

A user may subsequently input an intended Destination.

TVMDC will then calculate the best Course to Steer to take to get to the Destination; accounting for Set & Drift along with a Magnetic (TVMDC) bearing correction.

Simply put, Consider wind and current blowing you and drifting you off course? TVMDC is the App encompassing the techniques that a Sailor uses to put the craft back on track to the destination…

Quick Start: Using TVMDC

Download and install your copy of TVMDC on an iPhone device… Install TVMDC then Launch TVMDC.

TVMDC is used in 3 steps:-


Input the magnetic errors as:

  • Chart Variation (taken from the Chart’s Compass Rose)
  • Chart’s Annual Increase in Variation (taken also from the center of the Chart’s nearest Compass rose).
  • The Charts Year of publication. Displayed in the main Chart Title Box and sometimes in the Rose.
  • Ship’s Compass’s Deviation (written on your boats “calibrated” Compass)

A user now has access to instant corrections to a bearing of any type : True, Magnetic or Compass (T, M or C).

Simulator Screen Shot Dec 5, 2015, 12.17.46 PM


A Correction compass is also displayed to show your the three bearing types.

Simulator Screen Shot Dec 5, 2015, 12.17.56 PM

Note here, in the above graphic, as an example, the difference between your Boat’s compass reading and the True bearing, is 6.5 degrees.


The Set andDrift of a boat describes the motion of the Boat for if it was not powered. That is, the way the wind and the tide would take the boat , if were to drift.

Calculate the Set & Drift values at your current location. The Set (direction) and Drift (speed) values are those describing who current and wind is moving the boat, out-with your control.

While sailing along, take two fixes,  with their respective time stamps. Input the heading of the boat ( the direction the boat is pointing) and the speed through the water (as measured by your boat’s Paddle Wheel Transducer)

TVMDC will calculate the Set and Drift from these input values.

Simulator Screen Shot Dec 5, 2015, 12.19.31 PM


The calculated Set & Drift triangle is displayed.

Simulator Screen Shot Dec 5, 2015, 12.21.42 PM




Calculate your best course to steer to a destination in the presence of Set & Drift currents.

Input a desired Destination and Update your current speed through the water.

And that’s it.

Simulator Screen Shot Dec 5, 2015, 12.22.01 PM


TVMDC will calculate the best Course to Steer, the Estimated Arrival Time and the Velocity made Good (aka. Speed over Ground)

TVMDC will use your previously calculated TVMDC errors along with your previously calculated Set & Drift Errors to calculate the best Course to Steer (CTS).

A CTS sailing triangle is displayed.

Simulator Screen Shot Dec 5, 2015, 12.22.16 PM



Settings are also available to store the current quadrant that you’re sailing in.

Download the TVMDC App from iTunes…

For info needed about TVMDC?     Wikipedia background on TVMDC