Chart Plotting

Equipment

This is my own personal list of favorites for plotting positions on paper charts and coastal piloting. If you have GPS connected to a chart plotter connected to an autopilot while you lie in bed keeping watch using a video camera, just remember that all electronics break. A manual backup system is always useful.

Dividers

Create a Compass Deviation Table

A deviation table is important if you need to know the course to steer on open water. It will enable you to calculate the compass course if you know either the true course or the magnetic course from the course. Compass direction usually needs to be adjusted for magnetic interference on your particular boat.

Everything from metal, radios, speakers, engine, to beer cans can affect the compass needle as it tried to point to the North magnetic pole! Note that it may be necessary for sailboats to make a separate deviation table for when the motor is not running. A hand compass is very useful for making a deviation table for your ship compass as you can usually find a place for it without any interference.

  1. Find a place on your boat where you can sight straight ahead, free of all magnetic interference (electric wires, magnets, metal) for taking hand-bearing compass readings.
  2. Turn on all instruments you normally use when underway.
  3. Be sure to work in calm, open water.
  4. Turn the boat slowly onto all compass points in turn. As the boat settles on the points (0, 15, 30...) the helmsman calls "mark zero degrees". The person with the hand compass takes a comparison reading and writes it down.
  5. Enter these courses as 3 digit numbers (e.g. 090) in a chart with the hand-bearing compass measurement in the Magnetic column and Compass direction in the third column. The Deviation is the difference between your Magnetic measurement and your Compass measurement. It and is entered into the middle column. Write W (west) after the difference if the compass course is greater; E (east) if it is smaller.

Converting Chart directions to Compass courses

TVMDC is the magic formula for comverting True chart directions to compass directions. "Tough Virile Men Don't Cry" or "Tired Virgins Make Dull Company" may help you remember the order. Navigators used to all men!

For example 90 True (due east) on a chart is not the direction you steer using your compass since a compass points to the earth's "magnetic pole" not the north pole marked on a globe. Varation is the difference between these poles and is marked on every chart printed. If the chart has a stated variation of 15W then a True direction of 90 degrees is actually 105 degress Magnetic. In other words, you would have to head on a course of 105 degress to get where you are going.

If your chart Variation is west you add it, if it is east you subtract. "East is least and west is best" is a phrase to help you remember which to add or subtract when moving from True to Compass (left to right).

If the chart states that the Variation is increasing or decreasing every year, then you must calculate the Variation for the current year.. Multiply the number of years since that chart was printed by the yearly change and add or subtract it from the stated Variation. The Magnetic circle on the compass rose is only accurate for the year the chart was printed in this case.

Now you know which Magnetic direction but you still cannot use your compass to steer that course because all boats have some error in their compass. Everything from electric wiring, metals, radios, speakers, gear, engine, to beer cans can throw the compass needle off. This error is called compass Deviation and is different for every boat. You should check the chart after you install any new equipment that could affect your compass.

You must estimate values that are between others on your chart. For example, 105 degrees is not on your chart, but you will notice it lies between 094 and 120. Use the value between 4E and 5E that is the same distance between the values -- 5E. Now add or subtract using the rule to get your Compass course. This is the course you will steer to get where you want to go.

Example of a boat deviation chart from 0 to 115 degrees:

      Magnetic Deviation Compass
	355	 5W	  000
	008	 7W	  015
	028	 2W	  030
	044	 1W	  045
	060	 0	  060
	076	 1E	  075
	094	 4E	  090
	105...... ?
	120	 5E	  115

Example of converting a chart direction to a compass
course to steer by (ignoring currents and leeway):

	True	Var	Mag	Dev	Compass
	090	15W	105	5E	100
TOP back