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Go Back   SailNet Community > General Interest Forums > 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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  #21  
Old 01-29-2014
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Re: Magnetic Variance

Dean--

Don't make things unnecessarily complicated for yourself.

In general, magnetic variation arises because the position of the magnetic north pole of the earth does not correspond with the geographic north pole, which is centered at the axis of the earth's rotation. Hence, only on rare occasions will one's compass actually point "North" toward geographic north. Unfortunately, the variation of the alignment of the magnetic field at any given position on the surface of the earth is also influenced by local anomalies that arise due to the fact that the earth really isn't a smooth homogenous sphere but a rather a kind of lumpy round semi-sphere, squashed at the top and bottom, bulging in the middle; and, of inconsistent density. Factors that influence the shape of the magnetic field that surrounds the earth that your compass relies on.

Disregarding the foregoing, if you are located in a position where your chart shows a westerly magnetic variation, when your compass is pointing "North", it is pointing west of True North by the amount of variation to the west. Likewise, if you are in an area with an easterly variation, you are heading east of true north by the amount of the easterly variation.

For example, with westerly variation of 7º, if your compass is pointing "North" (0º or 360º), it also pointing 353º "True", relative to the grid on your chart (= 360º-7º). Likewise, with easterly variation, if your compass is pointing "North" (again 0º or 360º), it is really pointing 7º (= 0º+7º) "True". So, one "adds" east and "deducts" west.

Of course, one also has to account for "Deviation", the effect of one's ship on headings displayed on one's compass, which is unique to each boat and can only be obtained by "swinging" one's boat in a position with known true headings and making up a deviation table. With the deviation table, one adds or subtracts the known deviation on any given heading from the compass reading, then adds or subtracts the known variation, to get to one's true heading.

Got it?
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Last edited by svHyLyte; 01-30-2014 at 01:04 PM. Reason: correct typo
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  #22  
Old 01-29-2014
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Re: Magnetic Variance

Thanks for the replies everybody. Osprey, Donna is correct. I want to learn to get where I'm going without electronics. I have no problem with electronics and will definitely make use of them. The whole point is that if my boat suffers an electrical failure, lightening takes out the electronics, the batteries fail, or whatever, I will be able to find my way confidently.

Chapman's presents the subject a little differently, and maybe a little easier to understand. It says the 3 north's I need to know are true, magnetic, and compass north. It did explain better the difference in compass north and magnetic north. I'm in the process of reading it again. My printer is out of ink so that delays the idea of printing out a chart for now. I'm going to use the examples in Chapman's to write my own quiz questions and I'll post them on here to let the forum grade me and see how I do. I'm getting a wisdom tooth yanked tomorrow so it may be a couple of days before I get around to it.
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Old 01-30-2014
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Re: Magnetic Variance

Dean, we have this problem (Difference between mag north and true north) because we have a habit of drawing maps with north up however the magnetic north and south poles are the ends of the magnetic field around the earth. The magnetic field is created by magnetic elements in the earth's fluid outer core and this molten rock does not align perfectly with the axis around which the earth spins. Since we do have are maps with north straight up we have an ability to make the compass work with a map. Magnetic variation or declination is the answer. So depending on what point of the earth you are (or going) we have this east/west rule. In aviation we learned "east is least, west is best". So...if you have a true course of 100 degrees and you have a west variation of 4 degrees west... 100 degrees + 4 degrees = 104 degrees is your compass course. Remember that these is a difference between a course and a heading. Now with East variation you would subtract the 4 degrees for a course of 096 degrees. Now you need to add or subtract winds and water currents of your magnetic heading.

Hope this helps,

Jerry
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Last edited by jerryrlitton; 01-30-2014 at 06:56 PM.
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Old 01-30-2014
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Re: Magnetic Variance

Ok, I think I've got this. No problem remembering T V M D C. I'm using "West is best, East is least." I'm going to type some random givens, figure the rest, and let you guys tell me how I did. Laughter is accepted, name calling only if you correct me. Here goes....

Given: True bearing 284, Variance 2*W, Deviation 5*E

T 284* V 2*W M 286* D 5*E C 281*

Given: Magnetic bearing 357*, True bearing 005*, Deviation 5*W

T 005* V 8*E M 357* D 5*W C 002*

Given: Compass bearing 162*, Magnetic bearing 157*, Variance 23* W

T 134* V 23*W M 157* D 5*W C 162*


I see now why there are so many mnemonics floating around out there. You just have to find one that clicks for you!

Edit: The dentist rescheduled so I can work on this all day long! WooHoo!!
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Old 01-30-2014
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Re: Magnetic Variance

Good progress you've made! Your math (second line) is correct on all three situations, though I can't match the "Given" wording of numbers 2 and 3 with the TVMDC math part. But the math, in the correct order, is what matters.

Just bear in mind that when you go the other way, from Compass to True, that "East isn't least", it's the other way around--you add East ("at Elections") and subtract West error.

In practice, you usually are going from True to Compass anyway*, seldom do you need to do the reverse. But just sayin'...


*except maybe on a beat, where you tack and find out your compass course first, and then your poor navigator (meaning you??) have to convert to True for your semi-accurate plot of beating to windward. this is probably the most difficult type of compass-only conversion/plotting chartwork you'll have to do. Anything you can plot with only one course line (meaning reaches and runs) is way way simpler and more reliable than "naviguessing" a beat when you have no landmarks and no electronics. Which is why it's worth practicing, ha ha. And it's why we call this "dead reckoning".... ;-)

Last edited by nolatom; 01-30-2014 at 03:15 PM.
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Old 01-30-2014
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Re: Magnetic Variance

Dean I'll give you some easy rules to burn in your brain.
Up north the true north and magnetic north sit beside each other. From where you are, the difference is in degrees on the heading..
A saying is: "East is least. West is best." (least=minus) (best = add) on any map with true long. and lat. lines.
So "True heading and variance = magnetic heading."
If variance is east, you subtract the number from the true heading to get magnetic heading.
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Re: Magnetic Variance

I'm glad to hear I got the math right. I struggled with it for a while, which I'm sure you could tell from my posts. It just finally clicked. Once I got it straight in my head when to add and when to subtract, the rest was easy! When I wrote out the givens, I randomly picked numbers and even mixed up the order to make it more confusing. The variance, deviation, and at least one type of bearing should always be available.

I'm currently reading Navigation: A Newcomers Guide and the next subject is dead reckoning. I haven't made it through that part yet though.

Thank you all for your help. It's greatly appreciated!!
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Old 01-30-2014
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Re: Magnetic Variance

Quote:
Originally Posted by nolatom View Post
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.
when at captains school in ft lauderdale the lady teacher said it was

TIMID VIRGINS MAKE DULL COMPANY ADD WHISKEY

jajaja I see it can change a bit depending on who taught you, jajajaja
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Re: Magnetic Variance

GREETINGS EARTHLINGS , Now pay attention please I have had to teach youngsters this stuff while sailing a big gaff ketch (72 ft) and the best method to get this to stick in a tired mind is TO TRUE RIGHT ON (E) LEFT OFF (W) when you have done a double watch and it's getting dark and shallow you cannot afford to make a muksip Hope this helps AS ALWAYS GO SAFE
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Re: Magnetic Variance

here's another method:

1. Set your chartplotter to show Magnetic headings instead of True. Just follow those same headings on your boat's compass.

2. Print out this thread in case your plotter gives up and you have to go back to paper charts.

Seriously, I learned the TVMDC jingles and passed the nav exam, etc. etc. but I rarely use any of that now that there is a decent plotter on the boat. It speaks magnetic. All the compasses speak magnetic. No need ever to convert T to M or M to T. (Deviation is minimal on my fiberglass boat unless I park the binocs on the pedestal. Makes the compass lie)
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