VARIATION – DEVIATION – DECLINATION
Clear Terms for Clean Navigation
by Reinhard Zollitsch

Not all “experts” know better:

I recently found an article on navigation in a Canadian canoeing magazine. Various “experts” were asked how best to teach basic navigation to small boaters. The whole thing actually wasn't much more than explaining that a magnetic compass does not point to the actual geographic north pole, true north, but to the magnetic north pole, a tad to the side, and that thus every course steered by a magnetic compass has to be corrected for this “error”.

Most of us learned this fact in a Boy Scout orienteering workshop; we even learned that this “error” was called variation. It says so on every map and chart. There even are visuals, a compass rose, pointing to true north, the top of each map, following the north/south grid lines (meridians/longitudes), and a smaller compass rose, sometimes inside the bigger one, pointing to magnetic north from this particular geographic map/chart location (see picture).

compass
Chart compass rose for Penobscot Bay, Maine
(click to enlarge)

Let me jump ahead by stating that the terms deviation or declination are not depicting the same as variation and should thus not be used for the phenomenon described above. The “experts” in the Canadian magazine, however, used these three terms interchangeably, as if they were synonymous. So I asked the magazine editor to please correct this sloppy use of these three terms, while supplying them with a brief explanatory comment to appear in their next issue. It never happened.

I feel, however, that MAIB readers are much more knowledgeable as well as curious and eager to clean up these three terms, if they do not already know the difference. So here is my expanded explanation of these three terms.

Variation:

It is defined as “the angle between the true and the magnetic meridians”. That sounds easy enough, but what does it imply? The first thing one has to know about variation, as I see it, is that it varies from place to place, i.e. it depends on the specific geographic location you are at in relation to the magnetic pole.

If, for example, your geographic location on the globe is in line with both magnetic and geographic poles (i.e. is on the same meridian/longitude), the variation is zero, because at that point the compass needle points not only to magnetic north but also to the geographic, the true north pole. If, on the other hand, your location on the globe is at right angles to this specific meridian/longitude, the variation will be at its greatest.

Our next basic statement would then be that if the magnetic meridian/longitude is to the west of your true geographic meridian, we speak of a western variation, if it is to the east, we of course call it an eastern variation.

Looking at the magnetic compass rose on your map/chart, you can clearly see that you would have to ADD a western variation, but SUBTRACT an eastern variation when plotting a course, i.e. compensate for the compass error. Always remember that the chart course, based on true north, is the “real” thing, and to make your boat go “truly” north for example, you would have to “jerk” the compass needle to the right or left by the amount given in the variation for that particular location.

Example: In Maine we have about a 20 degree western variation. So to go north, you steer 20º on your compass; to go east, you steer 110º; south, 200º – get it? Always add the 20º variation to your intended course! Please do not make the terrible mistake of reversing the +/- . I have paddled with guys in Maine who stubbornly maintained they knew better and then ended up 40º off course!! Poor fellows! (The right way of figuring a course to the east is: 90º + 20º = 110 degrees on the compass; wrong way: 90º – 20º = 70 degrees, resulting in a 40º error!)

Deviation:

A lot of boaters, though, use the term deviation when they should be using the term variation. Deviation is an additional compass error caused by the ship/boat itself, not its geographic location, and varies with the course the ship/boat is steering. It is caused by the ferrous metals and electronics on board ship and is thus negligible for sea kayaks, canoes and small sailboats, unless of course you stowed your sardines and tuna cans directly under your compass :-)

Ships have a professional compensator swing the ship through a full circle with known fix points on shore and establish a deviation chart for which the ship's navigator again has to compensate by adding or subtracting the given values for a specific course. Again, it has nothing to do with the geographic location the ship is at (as variation does). Deviation is boat and course specific, and can totally be ignored by us small fry.

Declination:

The third term mentioned above, declination, is reserved for celestial navigation and astronomy. It defines “the angular distance of a heavenly body from the celestial equator”. And since most of us “Messers” don't do this sort of fancy stuff, you might park this term completely. I only vaguely remember it from my German high school spheric trigonometry class as well as my university course in advanced sailing navigation. I also recently came across that term again when I was reading up on diurnal and semi-diurnal tides. (I learned that the declination of the moon, its angle to the celestial equator, also affects the tides.)

Summary:

For simple small craft navigation by chart, compass and stopwatch (dead reckoning), only the term variation applies. It is important to know, though, that you should add a western variation and subtract an eastern variation from the true chart course, when laying a course.

Deviation is for big ships where on-board electronics and the steel hull itself would affect the magnetic compass.

Declination has to do with the celestial equator, a “heavenly” reference point. You thus never have to compensate for it. It does not affect your little Ritchie or Danforth compass in front of you.

I hope this clears up these three terms for a cleaner, less befuddling way of navigating.

© Reinhard Zollitsch


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