Compass Correction
As a quick review, to convert from compass heading to true and back again you can use these mnemonics.
Can = Compass Dead = +/- Deviation Men = Magnetic Vote = +/- Variation Twice = True (At Elections) = (Add East, Subtract West)
Our steering compass heading plus or minus our deviation on that heading equals our magnetic heading. The magnetic heading, plus or minus our variation, is our true heading. For all operations, add East, subtract West.
Going the other way, from a known true heading to the compass heading we need to steer, use this device:
T = True V = +/- Variation Makes = Magnetic Dull = +/- Deviation Children = Compass (Avoid Watching) = (Add West, Subtract East)
If we know that we need to steer a true course for a specific destination or next waypoint, we take that course, add/or subtract variation to derive our magnetic heading, and then add or subtract our deviation on that heading to obtain our compass course. For all operations, add West, subtract East.
All of this, however, presumes an accurate deviation table for the steering compass, derived by a professional compass adjuster using an alidade and other tools of the trade. On the other hand, verifying your deviation table at sea is good seamanship when everything is going well navigationally and absolutely critical when it isn’t.
Standard compass calibration by celestial objects while underway in open ocean, by azimuths or amplitudes of the bodies, should be attempted daily as weather permits. Here is a quick and dirty means of deriving a deviation table with no special tools and almost no math. It does require that you’re in the Northern Hemisphere, though, as we’ll be using Polaris to guide us. We’ll assume for this exercise that the azimuth to Polaris is close enough to true north to simply call it that. However, if you have the means to derive a more accurate
azimuth to Polaris, take advantage of that additional accuracy.
Traditional means of determining compass error from a celestial object presume the ability to sight the object over the compass using an alidade on all headings. Small-boat compasses, however, are rarely mounted in a way to make this possible. In this case, we’ll need to get creative.
Let’s assume that for our location, per our charts, our magnetic variation is 16° East. First, we point the boat directly at Polaris and read the compass. Our heading is 000° True (by definition), and our compass reads 335° Compass. Using our standard True to Compass formula, we see the following: 000° (=360°) True (Polaris) minus 16° East (variation) equals 344° Magnetic. Therefore, 9° East (deviation) must be subtracted from 344° Magnetic to equal 335° Compass. Now we have our first data point.
We then turn the boat directly away from Polaris and read the compass again, now on a heading of 180° True. From the data we collect, these first two readings will be the most accurate. On 180° True our compass reads 173°. 180° True minus 16° East (variation) equals 164° Magnetic, so adding 9° West (deviation) equals 173° Compass.
The fact that the reciprocal courses yield equal and opposite deviations indicates that we are doing our math correctly; however, an uneven distribution of east and west deviations may simply indicate a misaligned lubber’s line on our compass, and the table we are creating will naturally correct for that with no further calculations.
Next, we put Polaris on our port beam as precisely as possible and read the compass on a heading of 090° True, which yields 072° Compass. 090° True minus 16° East (variation) equals 074° Magnetic, meaning our deviation on this course is 2° East. We do the same on the starboard beam on a reciprocal heading of 270° True. Our compass reads 256° Compass. 270° True minus 16° East (variation) equals 254° Magnetic, so our deviation is 2° West.
Written as a table, our data looks like this:
We then transfer our deviations to graph paper. (Always carry plenty of graph paper; we use it for lots of things.)
At each of the major amplitude deviations (9° East and 9° West) I extended that deviation 10° of heading in each direction to better approximate a sine wave. Otherwise, I simply connected the dots.
Now, with this graph, we can easily determine our deviation for any true heading. In the drawn example, for a true course of 120°, our deviation is 2° West. To make 120° True, I subtract from it 16° East (variation) to give 104° Magnetic, and then add 2° (deviation) to give a course of 106° Compass to steer.
This may seem like a lot of math, but if you incorporate this into your daily watchstanding, even when you have all of your electronics, it will become routine and instinctive. Good watch!
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