Chart Plotting and other fun stuff

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.

• Straight leg dividers measure distances on a chart and store easily. Buy a good pair as you will have them forever.
• A rolling plotter is far easier to use than parallel rulers if you have to transfer a bearing to a compass rose on a small chart table when things start to pitch and roll.
• A magnifying glass is useful if your eyesight is not 20/20 to see those tiny symbols that might be ROCKS. Buy one of those amazing plastic magnifying "sheets" with the fresnel lens that magnifies everything under it. The nature store also had tiny clear plastic "cubes" magnifiers that are very handy. For some reason, it's difficult to find "drugstore" bifocal sunglasses in Canada for the over-40 boaters who are beginning to need reading glasses just to check a chart.
• On deck, a small spiral notebook with a pen clipped to is handy for making notations of time, distance, direction, and any navigational aids to be added periodically to your log for estimating position.
• A plastic chart cover protects your chart in the cockpit not only from rain, but from dirt and folding. Buy some clear plastic from the hardware store - measure your typical chart and double the shorter dimension (add several inches for extra space and "hem". Fold it and sew along the two short sides leaving the long side open. You can put two charts in it and fold them easily.
• A piece of masking tape cut in the shape of an arrow with the point coloured red makes a very nice chart marker on your new plastic chart cover. Use it to keep track of your progress as your move along courses with visual aids to navigation like buoys and lighthouses. They can also safely be used on strip charts.
• Good binoculars will get used often. Try 7x50 with coated optics for 7 magnification (higher is bad on a moving platform) with a 50 mm lens (a nice wide lens give a wide field of view). Waterproof is nice but adds a lot to the price (a dry location by the companionway makes them easy to use). Price is $100 to $700. You can get a build-in compass for about $300 up.
• A hand-bearing compass is nice to have for taking bearings on shore objects ($40 - $150). I prefer the kind that fits in a pocket and you sight through at your eye. It should have a light that does not require batteries.

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