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Magnetic Variance

9K views 68 replies 18 participants last post by  dabnis 
#1 ·
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.
 
#2 ·
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.
 
#4 · (Edited)
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?
 
#9 · (Edited)
Well since no one's said it, and at the risk of outdated PCness, I'll repeat the jingles I learned back in Power Squadron and around the docks: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.
 
#11 ·
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:...
:)

He was already thinking so much I could hear his brain cells popping. I was trying not to answer more than he asked.
 
#21 · (Edited)
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?
 
#22 ·
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.
 
#23 · (Edited)
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
 
#24 ·
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!!
 
#25 · (Edited)
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".... ;-)
 
#26 ·
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.
 
#27 ·
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!!:D
 
#29 ·
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
 
#30 ·
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)
 
#35 · (Edited)
I'm of the mindset that good seamanship includes being prepared as best you can. I will definitely be using a chart plotter but I will also most likely be singlehanding, or short handed at best. I don't want to be learning how to do this stuff in conditions that force me to use it. I see no sense in carrying paper charts If I don't know how to use them. Kinda like carrying a liferaft and not knowing how to inflate it. It sounds like it pays to minimize the deviation. I'll still be carrying reference books on the boat.

However when it is all said and done, the course and heading are two different things. Your heading is what you steer to arrive and the desired course. Has anyone mentioned compass deviation? (compass adjusted for magnetic anomalies caused by the boat) This really should have been the 1st step, then variation (declination), then heading (=/- currents and winds).?

Jerry
Thanks for mentioning it Jerry. I caught on to the deviation also. I already knew that course and heading are two different things. When I sailed my boat on Kentucky lake I had to allow for leeway and current. I just took a guess for my heading allowing for those and hope my course was close to what I wanted. In the short distances involved, that mainly came into play when making room for the occasional barge. I still need to learn how to more accurately account for them though.

Absolute simplest way to do it, IF your compass has almost no deviation?

Just lay your course out using the Magnetic compass rose, and steer magnetic.

Yes, you'll still have to adjust for leeway and current, but you won't have true virgins or dead men to deal with. ;-)
Good point using magnetic plots. True virgins and dead men could get a man in trouble in most parts of the world!:laugher
 
#31 ·
However when it is all said and done, the course and heading are two different things. Your heading is what you steer to arrive and the desired course. Has anyone mentioned compass deviation? (compass adjusted for magnetic anomalies caused by the boat) This really should have been the 1st step, then variation (declination), then heading (=/- currents and winds).?

Jerry
 
#33 ·
Once you figure your COA (course of advance, or desired course), remember just round off the the calculated course (true, but preferably magnetic, or if you have a deviation table, compass) to the nearest 5 degrees and steer that course. You cannot steer a course much more accurately than +/_ 5 degrees anyway, especially in rough conditions. Then...assuming a DR plot...

Fix your position as frequently as prudent, so you know where you are at all times (every hour, four times a day, daily, depending on the passage duration).

The navigator's adage is: the art and science of continually checking and calculating and fixing your position...and finding your errors (if any), before you have placed yourself and your vessel in serious trouble.
 
#36 ·
I've read in several places about the 5* thing. The compass on my old boat was marked in 5* increments. I noticed on the really windy days I tended to wander +/- 5* each side of my heading anyway. I'm thinking that keeping a DR plot charted as I go while at the same time charting my course by GPS will not only keep my mind actively engaged in where I'm at and where I'm going, it will also help me to improve my accuracy. I'm also planning on learning how to do celestial navigation later on and I'll check those positions against my GPS. Hopefully, if I'm ever in a position that my electronics get fried, it will be an inconvenience rather than something else to stress out about.
 
#45 ·
Dean--

I do not know the size of your boat or what room you have but a tool I have been using since the mid-80's, now on two different boats, is an inexpensive Plotting Board with a Pantograph fitted with an adjustable compass rose (see Weems & Plath Chartkit Plotter). With this, one can adjust the compass rose on the parallel rule for the amount of variation shown for each chart one might use and easily plot one's courses/headings in magnetic but easily translate them to True headings. The board can be laid out below, and, when necessary or desirable, taken into the cockpit (assuming one is using water proof/resistant charts). This equipment is very useful, particularly, for example, with Explorer Charts of the Bahamas, which report some significant differences in variation from locale to locale based upon local magnetic anomalies (which may account for some of the Bermuda Triangle mysteries). With this, you only need to work out your deviations and keep a little table, perhaps laminated in plastic, on a lanyard at your helm as we do.

FWIW...
 
#47 ·
I don't currently have a boat. I use to have an Endeavour 32 and I kept my chartbook in the cockpit with me. When I purchase my next boat, one of the features it must have is either a fixed table or preferably a dedicated nav station. That portable chartboard looks handy though. Even with having a nav station, I could see me using it in confined or crowded waters when I need to remain in the cockpit. Thanks for the tip!
 
#48 ·
As a resident of the True North Please allow me to extend an apology to all who struggle with magnetic variation .Apparently it's caused by the wandering magnetic pole up in NE Canada. Our noble gov has set the senate on the job of alignment with the true north pole but in truth they are not up to that task either .(see Peter Principle )
 
#50 ·
Rarely do I ever hear anyone give a position in anything but lat/long. In Chesapeake Bay people will often say things like "off Lookout Pt" or "a mile east of red 34" but never have i heard anyone give an exact bearing in either true or magnetic. It might be important in theory and yes, I learned to do it like everyone else but it just doesnt come up very often in real life unless you are still firmly in a paper chart only universe. Most of us arent any more.

Sure, it is a good thing to be ready for disaster to strike but do you REALLY keep a dead reckoning going on a long passage when you can simply read off the lat/long whenever you like and write it in the log? I think a lot of folks are piously repeating what their lessons told them was "proper" but which is a lot of work for a less accurate result.

It is a good idea to remember WHY our teachers told us to do all these calculations. It was to know our position with as much accuracy as possible. Again, I can do it, but why would I waste time with that when I have 4 redundant GPS's which will tell me my position with 10ft. accuracy? It is especially pointless when I am hundreds of miles from land and seem to only rarely have a wind fair enough to head directly toward my goal.
 
#52 ·
I see the point you are trying to make but it is still a good thing to know. I happen to like paper charts. If "most" others don't, well, that's all well and good but has absolutely nothing to do with me or my own preferences. I also like the accuracy of GPS. I think they both have their place. I also think it a wise precaution to keep a track on a chart and knowing how to do these conversions is necessary in order to do that. You said you can do these calculations. Well, I'm just learning. As to the "why" I want to learn them, I explained that in my previous posts. Anything will be pointless if you see no value in it.
 
#53 ·
Thank you all who are helping to agree that it is OK to use all best knowledge, and technology we have for safe navigation. Most of time as beginner I am staying very close to visual reference of known land. Due to lot of factors this can be confusing.
Especially when judging distances over water. In this case charts, and last known position on it clears confusion between visual reference, and what chart plotter is showing. Practice, practice, and enjoy doing it especially in fair weather, as they say.
 
#54 ·
You're right Osprey. It's all good. I think it really falls back to personal preference. I like being able to look at a chart and letting my eye pick out the details I need at the moment from within the big picture. Chartplotters get expensive as the size of the screen goes up. For me, even the big ones I've seen lose the fine details if you zoom out, leaving you the choice of looking at either the big picture OR displaying the details, but not both. Some people may be able to get what they need off of those small screens but not me. I just prefer a chart.

My sailing was done on Kentucky lake and at no time was I out of sight of land. I carried my handheld Garmin GPS but did practically all navigation with my chartbook, a compass, and my eye. The GPS came in handy at night or during foggy conditions. I wouldn't want to be without either.

If you don't deal with measurements on a daily basis, I would suggest getting a cheap rangefinder. They are pretty accurate out to 500 to 800 yards on a reflective surface. Once you get good at judging distance out to 100 yards you can use simple addition to count how many 100 yd increments are between you and your target. If you keep an accurate position of your boat, just pay attention to how far an object appears to your eye, then measure it on your chart. After some time, your eye will become more accurate. Hope that helps.;)
 
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