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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 03-10-2014
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Variation question for chart navigators

Hi All,

Something I've wrestled with for a long time... the annual increase/decrease of variation and how it applies to my chart work.

I'm studying the book "Inland and Coastal Navigation, 2nd edition" and it says "The additional note telling how it changes with time, such as annual decrease 6', is usually of little practical interest".

Would I get terribly chastised for not taking into account that bit of information on a 3 year old chart (or admitting to not doing so)? Thoughts on the subject?

Dave
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Old 03-10-2014
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Re: Variation question for chart navigators

Dave - on a coastal navigation exam it would be unwise not to account for changes in variation. On older charts and in areas where there a big annual change it should be considered. Check the date on the compass rose of your charts; it will likely be different from the date of purchase and the date of the last update from Notices to Mariners.
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Old 03-10-2014
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Re: Variation question for chart navigators

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Originally Posted by Dave_E View Post
...I'm studying the book "Inland and Coastal Navigation, 2nd edition" and it says "The additional note telling how it changes with time, such as annual decrease 6', is usually of little practical interest".

Would I get terribly chastised for not taking into account that bit of information on a 3 year old chart (or admitting to not doing so)? Thoughts on the subject?
The "practical interest" comment is spot on...if the variation is such that it affects your practical navigation...that is - when the product of the variation times elapsed time (in years) since chart construction is great enough to make a difference in your plotted bearings and courses...then you need to accommodate the change in deviation when using the protractor and compass and parallel ruler.

If not...for example the variation of 6' in the note would cause a deviation change of one degree over ten years...and few of us have plotting equipment or measuring equipment with high accuracy...then the variation can be ignored for practical purposes.

In computations on exams and in study guides you probably will be expected to apply the variation to solutions to demonstrate an understanding of it.
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Old 03-10-2014
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Re: Variation question for chart navigators

Like Fryewe says, your navigation equipment - and steering- probably isn't accurate enough to make a difference if you even try to account for the small changes called for in such a situation. The line you draw on the chart might be a degree or two wide, and can you steer exactly to it? And you're worried about x MINUTES a year change? If you've got a 10 year-old chart with 6 minutes a year change, there'll be a one degree difference in your course. Then you'll have to take leeway and current into account, so it'll be inconsequential. Why are so many charts still based on soundings from 1840? It doesn't really make that much difference most of the time. If someone goes aground, they'll re-sound it. If annual compass variation does make a difference, you will be able to figure that out, since they tell you what the variation is.

Last edited by paulk; 03-10-2014 at 10:18 PM.
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Old 03-11-2014
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Re: Variation question for chart navigators

Thanks for the insight guys. I was leaning toward that answer, but it helps to get others take on the subject. Exams... every ' counts, got it.

Dave
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Old 03-12-2014
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Re: Variation question for chart navigators

My perspective, FWIW, is a little different. It's true that the amount of change in variation won't matter much given the various limits on precision cited above. (Although the more times you use a slightly inaccurate course, the more significant the inaccuracy can become). But to me, there are reasons why it makes sense to figure the change in, always.
First, it's very easy--multiply the annual change by the age of the chart and add/subtract it to the variation number. It becomes second nature.
Second, doing automatically it is easier than making a conscious decision, in every instance, whether to do it or not. Like many other aspects to managing a boat, having standard procedures has many advantages. Not least of which, it makes sailing more relaxing when you have fewer judgment calls to make, and potentially fewer mistakes. And there's satisfaction in knowing a 'right' way to do things like navigation (and your own right way can differ from mine), and getting good at doing them that way.
Another reason: it makes you look at the age of your chart, which is obviously important for other reasons, and you might not otherwise.
John V.
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Old 03-12-2014
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Re: Variation question for chart navigators

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Originally Posted by VallelyJ View Post
Another reason: it makes you look at the age of your chart, which is obviously important for other reasons, and you might not otherwise.
John V.
And how many folks read the title block?

Most Canadian charts are metric, but some are still in fathoms.

Most are Mercator, but some are polyconic.

In the PNW, chart datum changes as you cross the border.
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Old 03-13-2014
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Re: Variation question for chart navigators

It's just a good idea to be in the habit of correcting for the variation. You may notice that the variation on the compass roses differs even on the same chart. This is of course because of the shifting of the magnetic poles. Even If you are not relying on paper charts to plot courses, you should at least correct for them as a backup in case you run into some kind of power failure.


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Old 03-14-2014
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Re: Variation question for chart navigators

After reading the recent posts above about annual variation, it becomes obvious that we need to include the earth's "wobble", or axial precesssion in our calculations when we plot our positions too. Astronomers have known about this from the time of Ptolemy, so don't be surprised. The currently accepted value is 50.2 arc seconds per year. If you make this correction each time you plot your position (do you do that every hour on the hour, or on the quarter-hour?) you won't have to worry about whether to do it or not. It will become automatic, and you won't ever have to worry about your plot being off by 3". You'll be able to sleep at night, not worrying about it. If we want to do things right, let's make sure we do it right. We can also rest assured that in 26,000 years the precession will circle us back around, and we'll be able to use the same calculations over again and be confident that we were right the first time.
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Old 03-15-2014
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Re: Variation question for chart navigators

Quote:
Originally Posted by paulk View Post
After reading the recent posts above about annual variation, it becomes obvious that we need to include the earth's "wobble", or axial precesssion in our calculations when we plot our positions too. Astronomers have known about this from the time of Ptolemy, so don't be surprised. The currently accepted value is 50.2 arc seconds per year. If you make this correction each time you plot your position (do you do that every hour on the hour, or on the quarter-hour?) you won't have to worry about whether to do it or not. It will become automatic, and you won't ever have to worry about your plot being off by 3". You'll be able to sleep at night, not worrying about it. If we want to do things right, let's make sure we do it right. We can also rest assured that in 26,000 years the precession will circle us back around, and we'll be able to use the same calculations over again and be confident that we were right the first time.
So if I fail to account for this, I could be off by a factor of 1/26,000th?

Very scary indeed. I'm convinced ;-)
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