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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 01-29-2014
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Question Magnetic Variance

I've been reading up on charts and how to plot bearings. The books I have all seem to describe converting magnetic bearings to true bearings in the same way and they have me confused.

I understand that the variance given in the compass rose has a stated variance, when it was measured, and the amount and direction of change per year. I understand how to find the current year variation from this information. What's confusing me is the statement "To convert a magnetic compass reading to true to plot on a chart *subtract a westerly variation *Add an easterly variation."

Assuming that my variation after figuring for current year is 5* W. and forget deviation for a moment, let's say I take a compass bearing of 270*. Would my conversion be 270 - 5 = 265*T? If the variance was 5* E then the conversion would be 270 + 5 = 275*T?

I'm not sure why I'm having so much trouble picturing this in my head.
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Old 01-29-2014
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Re: Magnetic Variance

Your variation depends on which side of the country you are on. In the U.S., for me, it will be west. For the west coast (and west of the isogonic line) it will be east.

But yes, you are correct. Magnetic to true you subtract westerly variation and add easterly variation.
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Re: Magnetic Variance

!!!Except in the Bermuda triangle! Or the superstition mountains, but you can't sail there.
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Re: Magnetic Variance

Quote:
Originally Posted by DRFerron View Post
Your variation depends on which side of the country you are on. In the U.S., for me, it will be west. For the west coast (and west of the isogonic line) it will be east.

But yes, you are correct. Magnetic to true you subtract westerly variation and add easterly variation.
The examples I'm looking at show the compass rose of a chart with the variance noted in the center. I was hoping that following the rule of add east, subtract west will work on any chart.

Another question; When the rose has an inner magnetic rose, it is drawn to the variance noted in the center for that year, right? If the chart is older and the variance over the years was great enough to change the degree variance, I'll still have to allow for that if I use the inner magnetic rose.

Thanks for the response by the way.

Edit: And I guess I reverse the rule when converting from true to magnetic?

Last edited by Dean101; 01-29-2014 at 05:07 PM. Reason: Additional question. I'm full of em!
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Re: Magnetic Variance

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Originally Posted by desert rat View Post
!!!Except in the Bermuda triangle! Or the superstition mountains, but you can't sail there.
I've heard that with all the magnetic disruptions in the Triangle, you can only steer a straight course while drunk on Pusser's rum and you have to create a deviation card for any other rums consumed.
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Re: Magnetic Variance

Quote:
Originally Posted by Dean101 View Post
The examples I'm looking at show the compass rose of a chart with the variance noted in the center. I was hoping that following the rule of add east, subtract west will work on any chart.
Charts for the west coast will have an easterly variation noted in the middle of the compass rose. Charts for the east coast will have a westerly variation. They aren't interchangeable in the way I think you mean. If you're using a chart for say, the entrance to New York Harbor, you'll use the westerly variation as noted in the compass rose.

They give you both east and west so that the publishers can use the same example on either coast. But you need to use the one that's applicable to your area.

Quote:
Originally Posted by Dean101 View Post
Another question; When the rose has an inner magnetic rose, it is drawn to the variance noted in the center for that year, right?
Yes.

Quote:
Originally Posted by Dean101 View Post
If the chart is older and the variance over the years was great enough to change the degree variance, I'll still have to allow for that if I use the inner magnetic rose.

Thanks for the response by the way.
You'll have to account for the change in variation over the years regardless of which direction you are going (magnetic to true or true to magnetic).

I hope this helps.

A great book that I recommend is How to Read a Nautical Chart by Nigel Calder. Good explanations and it also contains all of Chart No. 1.
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Re: Magnetic Variance

Quote:
Originally Posted by DRFerron View Post
Charts for the west coast will have an easterly variation noted in the middle of the compass rose. Charts for the east coast will have a westerly variation. They aren't interchangeable in the way I think you mean. If you're using a chart for say, the entrance to New York Harbor, you'll use the westerly variation as noted in the compass rose.

They give you both east and west so that the publishers can use the same example on either coast. But you need to use the one that's applicable to your area.



Yes.



You'll have to account for the change in variation over the years regardless of which direction you are going (magnetic to true or true to magnetic).

I hope this helps.

A great book that I recommend is How to Read a Nautical Chart by Nigel Calder. Good explanations and it also contains all of Chart No. 1.
Thanks, I'll check that book out. I just assumed that if I buy charts for any given area, the variance would be noted on it whether it was E or W. Most of my navigating has been piloting by landmarks or by GPS. I'm trying to self-teach myself on the subject. Using electronics isn't all that difficult but I want to learn how to do it without having to totally rely on them. Would you recommend Calder's book as a good source to learn not just how to read a chart but how to figure course, set, drift, accounting for currents, tides, etc. and all those other things that I should know but don't yet know that I should know?
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Re: Magnetic Variance

Quote:
Originally Posted by Dean101 View Post
Thanks, I'll check that book out. I just assumed that if I buy charts for any given area, the variance would be noted on it whether it was E or W.
It will be. I'm sorry. I thought I was more clear.

Quote:
Originally Posted by Dean101 View Post
Most of my navigating has been piloting by landmarks or by GPS. I'm trying to self-teach myself on the subject. Using electronics isn't all that difficult but I want to learn how to do it without having to totally rely on them. Would you recommend Calder's book as a good source to learn not just how to read a chart but how to figure course, set, drift, accounting for currents, tides, etc. and all those other things that I should know but don't yet know that I should know?
I don't remember offhand if it teaches you how to figure out your course with current set and drift. I think I left my copy on the boat in my nav bag. Boater's Bowditch has a brief explanation.
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Re: Magnetic Variance

Well since no one's said it, and at the risk of outdated PCness, I'll repeat the jingles I learned back in Power Squadron and around the docks:

To get from From True to the Compass course to steer, you add Westerly error and subtract Easterly error. So, it's True, + - Variation = Magnetic, which + - Deviation =
Compass (and you add Westerly error and subtract easterly error for both variation and deviation, so in shorthand it's:

"TVMDC--Add W", (meaning True, Variation, Magnetic, Deviation, Compass, and add Westerly errors)

How to remember?? as the old non-PC sailors said,

"True Virgins Make Dull Companions--Add Whiskey"


And to go the opposite way, from Compass to True, it's ("CDMVT, Add E") or the jingle"

"Can Dead Men Vote Twice--At Elections?" (here in Louisiana, of course the answer is yes, ha ha)

There may be a more modern set of jingles, but True Virgins and Dead Men is what I grew up with, and I can remember the jingles when I can't remember the formulas.

Last edited by nolatom; 01-29-2014 at 05:46 PM.
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Re: Magnetic Variance

Quote:
Originally Posted by DRFerron View Post
It will be. I'm sorry. I thought I was more clear.

.
You probably were. I've just been thinking on it for too long and didn't catch it. Thanks.
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