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Old 10-31-2000
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Jim Sexton is on a distinguished road
Compass Rose Navigation

On a vessel with magnetic compass only, I see no advantage to converting bearings to true before plotting since this is what the compass rose does. Do you agree?

Jim Austin


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Jim Sexton responds:

You are absolutely right about using magnetic courses and bearings via the compass rose. If you are comfortable doing all your work in magnetic, then keep on doing it. I also applaud you for doing the navigation work at sea, since you are an exception. I'm sure that the effort you expend today will be returned tenfold in the future. I would only emphasize that you label your courses, bearings, etc. with the upper-case M to show anyone else who may look at your work that you are using magnetic versus true. Not to use the M label implies that true is used.

My only objections to the use of the compass rose is the requirement of using a rolling ruler or parallel ruler and that often the rose is not in a convenient location whereas the latitude and longitude lines are usually nearby. If not, the compass rose can also be used for true. On a small vessel with an even smaller nav table, trying to find a rose or nearby latitude or longitude line from which to plot can often be a bit of a hassle. The magnetic to true computations, while not especially difficult, can be subject to error when you are stressed out or seasick. This is why using a nav log and recording the conversion process from true to mag or mag to true can help to prevent math errors.

Having said all that, let me explain why the true method is taught at the USNA, USCGA, and USPS, among other institutions, and used by all professional navigators in the military and merchant marines. To a large extent, it is a tradition having been done in that manner since before the time of Columbus. Also the main reference textbooks for navigation, Bowditch, Dutton's, and Chapman's all teach and recommend the use of true versus magnetic. They do this because using magnetic requires that all directions be plotted using magnetic—courses, ranges, bearings, currents, etc. If not, you or your replacement could get confused. During a watch changeover someone else might be required to pick up the navigation from where you left off and everyone concerned should be using the same plotting techniques. This is a very valid argument on a large, crewed vessel, but I realize that it is a moot point where you are the only navigator aboard a recreational vessel. Also these basic coastal piloting courses are preliminary to learning celestial navigation where ZNs (the bearing of the celestial body from true north) are in true. To plot magnetic here would require the ZNs to be converted from true. So as a matter of being consistent, true is taught and used for all marine navigation. Perhaps you have heard the old expression about the USN: "Five hundred years of tradition, unhampered by progress."

Now that more recreational boaters are learning coastal navigation, they are forced to learn the true method because that is what is required by the USCG Auxiliary and US Power Squadron policies. Since civilians cannot be forced to use traditional methods, they will always use reason and logic to question these techniques. I always tell my students that once they learn to use the true-course method and can pass the final exam, they are free to use magnetic, if that works best for them, as long as someone else can go back over their work and figure out what they did. If you are ever involved in an accident at sea, representatives of the insurance companies will want to see your chart and nav log to replot your vessel's movements. If they can do that easily, it may help to prove your side of the story and perhaps lessen your liability. Remember, there are no innocent parties when a collision occurs.

Marine navigation, just like seamanship, has many traditional methods of doing everything aboard a vessel. To a non-mariner some of them seem down-right silly, like calling ropes, lines or sheets. When you look at a three-masted schooner, do you realize that every mast, sail, line and sheet has a specific name? In the old days, students were required to identify all of them from memory before they could even set foot on board. Anyway, the bottom line is that some things are done that way because of tradition. Doing them so and doing them well will set you apart from your landlubber friends. It will also mark you as a professional.

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