Magnetic North VS True North Vs UTM Grid North

Factual Questions
1

I’m not sure why I’ve never asked this on here. I first thought to ask this over a year ago.

Earth’s magnetic pole is off slightly from the actual geographic norh pole. Depending on where you are, “North” on your compass will be several degrees off from North on your map.
I’ve always heard stuff about the Earth’s magnetosphere and how it fluctuates and one day it will actually reverse itself again and stuff like that. The compass doesnt point exactly north, because the earth’s magnetic poles are off slightly. This makes since to me.

However:

Every single Army course I’ve taken that involved land navigation, blames this phenomenon to some gigantic iron ore deposit somewhere in the north eastern United States (or around there somewhere). Until I joined the Army, I never heard anything like that!! But every single soldier seems to accept this as truth, and every instructor I’ve had repeats this “fact”. That the compasses are being pulled off of North because they are pointing toward this Iron Ore deposit.
And their rationale for the GM Angle changing every couple years, is because the Ore Deposit is being mind and it’s getting smaller each year. This fact was even repeated at the premier Army land navigation course, The STAR.

How much truth is there to this? I thought the compass WAS pointing to magnetic north, but because the Earh’s magnetic poles dont line up exactly with geographical north, we have a discrepancy on the map. But the Army is convinced this is due to an Iron Ore deposit in the US. Does this mean there is little or NO GM Angle difference in other countries???

I have grown to the point where I never believe ANY “fact” unless it’s been confirmed or given by several collaberating Dopers.

2

I call bullpoopies on this one. The magnetic poles are indeed mis-aligned from the rotational pole, and they drift around randomly. There is no other data needed to explain the difference in heading between the axis pole and magnetic pole. I’m pretty sure iron ore is non-magnetic anyway, though I could be mistaken on that one.

3

You must have misunderstood the content of your Army courses on navigation if this is what you got out of them.

4

This will answer a lot:

Magnetic Declination FAQ

Magnetic anomalies do affect compasses.

5

Absolutely NOT! Had I heard it only once, I would have brushed it off as a misunderstanding or that possibly that particular instructor was misinformed. I believe the first time I heard this was at Basic. And I’ve heard it countless times since then.

From the link above:

(Bolding Mine)
Ok… but does an ore deposit somewhere near Canada really create an 8 Degree GM angle in North Carolina?? If so, what is the name of this Ore Deposit, and where is it exactly?

One learns the hard way to keep his rifle away from his compass!!! This part says things will induce an error or 3-4 degrees. GM angles are upwards to 10 degrees or so. Or, if you’re in some really odd places as shown from the link, 90 degrees!

The GM angle in North Florida is like 4-5 degrees. It’s 8 in Fayetville, NC. And this is because of an ore deposit in Southern Canada/ Northern US???

6

Either you misunderstand or the course is talking rubbish.

The magnetic north is determined by the iron in the core of the planet, it has nothing to do with ore deposits on the Earth’s surface, which are miniscule (to the nth degree) by comparison. While they may cause local anomolies, they have no influence on the position of the pole itself.

7

No, the primary source of declination is the fact that poles of the Earth’s magnetic field, generated in the core, do not coincide with the axis of rotation. Local anomalies can affect declination locally.

8

I don’t think I’ve heard this before, but it occurs to me that maybe what the instructor was saying was that the magnetic north deviated from true north because of the gigantic iron ore deposit… which seems hard to prove but not exactly far-fetched.

When you think about it, the magnetized iron core of the planet is like a magnetized needle, suspended within a spinning spherical cage. Any other ferromagnetic mass of significant mass is going to affect it somewhat – like when you place a magnet near a heavy iron bar… the magnet pulls the bar towards it, and the bar also pulls on the magnet.

And for the record, all iron is ferromagnetic, which means it may not be strongly magnetic, but it will react within a magnetic field. (The magnetization of iron ore due to the magnetic field of the earth won’t be too strong based on any individual sample, but the effect of an entire huge deposit is something I wouldn’t discount.)

9

So you did misunderstand.

The general variation of the compass is caused by the dipole field of the earth being misaligned with true north. There are also other, higher order fields which result in localized kinks in the magnetic isogons. In addition, if the compass is near a magnetic material it will be affected, so yes, a large deposit of ore if it is ferromagnetic would locally cause a compass deviation. However, the army training didn’t say that was the cause of the overall, worldwide variation of the compass. So you did misunderstand, I guess.

10

The ore deposits of Iron Mountain, Michigan are pure enough and extensive enough to affect a compass even at some distance. However, that in no way accounts for magnetic deviations in other parts of the world.

11

Who’s the one misunderstanding???

I have been told repeatedly by instructors that the declination in FL, GA, and NC (only states I’ve trained in) is caused by a massive Iron Ore deposit somewhere in the Northeastern US near the Canadian border.

This is false! I’ve always felt it was crap. I winced everytime I heard it, but the Army is not known for being too enlightened, so I ignore it and drive on. I just wanted proof that was I knew was nonsense, was in fact, nonsense.

From what’s been said here, anamolies can effect the declination locally only. But nothing is going to throw something off a dozen states away!!

So tell me again exactly what I misunderstood!!

12

This could be the basis or their assertions. Do you know of any cites that claim this will effect a Compass in NC? In FL?

13

David Simmons,
If I’m sounding like a jerk, sorry. I’m not meaning to come off that way. I am just trying to find the facts here. And one fact I’m clear on, is that I did not misunderstand what I was told.

I’ve been specifically told that "a compass does not point to the North Pole because it is drawn toward an Iron Ore deposit up North.

The first part of this statement is blatantly false. A compass does not point exactly north because the magnetic poles are not lined up perfectly with the geographic ones. It wouldn’t matter if there was an ore deposit or not, the compass would never point perfectly north.

The second part of the statement is what strikes me as nonsense and what brings me to this thread. Is there an ore deposit up north (Iron Mountain, possibly) that is effecting compasses throughout the country? If so, specifically which ore deposit. What’s it called, and where is it?

It is a fact that the declination on a map will change over the years. I have been told this is due to the mining of the ore deposits. The deposit is getting smaller, and this is decreasing the effect on the compasses.
The answer to this depends solely on that of the first questions. However, since the earth’s magnetic field fluctuates regardless of the presence of superficial ore deposits, I believe this is wrong too.

But believing is not “knowing”.

14

OK, then someone was pulling your leg. The army as an organization has no incentive to teach a bunch of bullshit to people who might need navigational skills to carry out a vital national defense mission. Ergo, the army instructional syllabus for navigation wouldn’t have included what you were told by whoever did so which leaves the idea that someone was stringing you along.

Here is a NASA site on the magnetic field. The origin of the field isn’t understood in any detail but seems to be nailed down in principle. The field does not exist because the core of the earth is iron. The iron in the core is at a temperature, the Curie point, well above the point at which iron loses its magnetic properties. A table of Curie points, or temperatures, for various materials is in this site. Note that the temperature for iron, Fe, is 1043K or 770C, or about 1420[sup]o[/sup]F which is below the melting point for iron.

It is thought that the field originates because the core is a conductive liquid, molten iron, that is in constant motion because of convection currents that are altered by the fact that the earth rotates. The motion of the liquid in the core is so complex that no formula can really do more than roughly approximate it and this complex motion creates the field. So the field is also complex and consists of a dominant dipole field having two poles, one north and one south, that is tilted with respect to the earth’s axis by about 15[sup]o[/sup] or so. There are also many other subsidiary field components with multiple poles and the whole assembly gives rise to the total field.

Local magnetic materials can affect the reading of a compass. On ships and airplanes the magnetic compass must be calibrated and a compass correction card is located right by the compass so that the individual doing the steering can know what compass reading to use for a given magnetic heading that is wanted. Of course, nowadays, that isn’t the case because everyone navigates by GPS, but I’ll bet there is a good old magnetic compass somewhere around, just in case.

However, mountains of ore in Quebec, or Upper Peninsula, MI or northern Minesota do not, repeat do not account for the variation of a magnetic compass from true north.

15

No.

Sorry, I had to cut that post short last night because we had a tornado in the neighborhood and I had to duck-and-cover.

Iron Mountain will, as I said, affect a compass in the Upper Pennisula of Michigan and the part of Wisconsin the deposit runs through. It doesn’t, however affect a compass even as close as where I live, in Northwestern Indiana.

Sounds to me like someone heard about magnetic declination, its tendency to wander, a big iron deposit “up north” and got somewhat confused. Once it’s in a textbook, the confusion becomes perpetual. Assuming, of course, someone wasn’t just yanking your chain.

The magnetic poles actually wander around on their own. In air navigation, the information for using instrument approaches to airports not only list the magnetic declination for a particular airfield, it also lists it’s rate of change per year. So someone is out there studying this phenomena and tracking it.

There is a big anomoly under the south Atlantic Ocean - I’m not exactly clear on what’s going on, but it was explained to me as a region of the Earth’s magnetic field where the polarity is either reversed or the field is very, very weak (This is not one of my big areas of knowledge). I was also told it was very recent, and by that I have no idea if they mean 10 years old or 100 - nor do I know if it would affect the southeast of the United States.

It is the law that all aircraft must carry an honest-to-goodness compass, along with compass card. GPS only works as long as you have electric power, the compass is your unpowered backup. Also, the GPS system has occassionally gone on the fritz or been unaccessible - from time to time the FAA puts out a Notice to Airmen that the GPS system will be unreliable either overall, or in a certain region. Why they don’t always say, although I remember one occassion where the government was testing potential jamming for GPS and warned pilots in a large area of the West that they might have problems with their GPS during the test.

And, just for the record, Mr. Simmons, not everyone is navigating by GPS - Pilots are still required to learn to old map-and-compass navigation techniques, and quite a few of us still don’t own a GPS. Airlines, of course, are using it routinely.

16

Just wanted to add the following tidbits:

  1. It is incorrect to say, “A compass points to true north.” Likewise it is incorrect to say, “A compass points to magnetic north.” A compass simply aligns itself with the static magnetic field that happens to be passing through its needle. The compass cares not where this magnetic field came from.

  2. The magnetic field that passes through a compass’s needle is the vector sum of everything that produces a static magnetic field.

17

My faith in at least some small part of humanity is restored.

18

You might think this is a nitpick – a little like saying that even a stopped clock is right twice a day – but if you draw a great circle line thru both the magnetic and geographic north, anyone standing on that line anywhere on the earth will have 0 degrees in declination error (=none), subject to local anomolies, of course. So that person’s compass would indeed point to true north.

This line presently passes thru the US Midwest including Illinois.

19

Slight hijack:

Each topo map has an arrow signifying the direction of magnetic north. How, exactly, is this determined? I can think of two ways to do it:

  1. Make the arrow point to the geographical location of “magnetic north,” which is northwest of Hudson’s Bay in northern Canada (approximately 700 km [450 mi] from the true north pole). Note that a compass is not needed to do this. Any map software should be able to di it.

  2. Go to the location at the center of the map, and use a compass to determine the difference between true (geographic) north and magnetic north.

Does anyone know which technique the mapmakers use?

20

Thank you all for confirming what I thought I knew.

And as far as them ‘pulling my leg’, I only wish that was the case. But these people honestly believe this. Though it is not in any book or instructional material, it gets passed from person to person. Everyone “knows” this fact, so no one doubts it.

You have to understand that when teaching a bunch of Army guys about navigation, it is not necessary to explain the Earth’s magnetic field in detail. All they explain is that there is a difference in grid and magnetic north, and this difference is shown as the GM Angle on the bottom of the map. It takes all freaking day to explain to people how to convert a magnetic azimuth to a grid azimuth and vice versa. IT’S ONLY ADDING OR SUBTRACTING FOR CRYING OUT LOUD!!!

Anyway, no instructor has ever failed to include the little trivia about the iron ore deposits. I have no idea where this crap came from. But you see, someone told them that, so they are repeating it. And sitting in a class of 200 people, I am betting 199 (all but me) believed that crap. And in the future, when they teach a land navigation class, they’ll mention the same nonsense they learned about some iron ore deposit.

Ahhhh Urban Legends at their worst. It only takes one instructor to spew one line of bullshit, to start an unstopable chain reaction. It is engrained in a soldier’s mind that the instructor knows all. Combine this with an overwhelming ignorance across the board and you end up with stuff like this.