Compass headings vs VOR radials

[mstram]

I'm trying to relate the VOR radial "rule of 60" to magnetic compass headings.

I *think* the thing about the operation of the compass, is though the Earth has a magnetic "north pole", the magnetic field is obviously not just concentrated on that spot. (Like a theoretical VERY long range VOR would be).

Since the field is all over the world, I think the north attraction just gets stronger right at the pole.

So that's why there appears to be a "local north pole" wherever you are.
E.g. you're on the rwy taking off, a 1 degree change seems to relative to your previous position, and NOT the Earth's magnetic pole.

At least that's my theory.

It was never really discussed in ground school, and I haven't found similar info yet on the net.

E.g. I setup the garmin simulator with the plane at 0 lat, 0 long, then plotted a direct-to 75-46N 99-46W (current magnetic pole lat/lon).

The GPS says the DTK is 353M. Dist is 5544 nm

Then set the heading to 270 and speed to 600 kts

With the "rule of 60", 5544 / 60 = 92. Therefore after flying ~92 nm west, the DTK should change from 353M to 354M.

It actually happened some distance after that.

So that is "VOR like" behavior.

On the other hand, you can obviously change your heading at any time by a degree, and it doesn't take anywhere near ~90 miles to take effect.

Hence the "global-north-pole-everywhere" effect ????

Better, correct explanations gladly accepted :)

[Andrewgr2]

According to Wikipedia, in 2015, the magnetic North Pole is at 86.3N, 160W. Where does your position come from - and does thAt affect your calculation?

[Goldenrivett]

Hi mstram,
Your "rule of 60" is only an approximation.
The magnetic declination (variation to us in UK) changes all over the world, so you must take account of that change when you move to a different position.

See:https://www.google.co.uk/search?q=magnetic+variation+map&espv=2&biw=1280&bih=656&tbm=isch&imgil=jBd3SuJoET4JCM%253A%253BWG_SloDP7NPn5M%253Bhttps%25253 A%25252F%25252Fen.wikipedia.org%25252Fwiki%25252FMagnetic_de clination&source=iu&pf=m&fir=jBd3SuJoET4JCM%253A%252CWG_SloDP7NPn5M%252C_&usg=__xwSZBAdxFiy1pfFmFNsdjb326aw%3D&ved=0CC0QyjdqFQoTCOvhrvG45McCFUOn2wodby8LeA&ei=pULtVav7BcPO7gbv3qzABw#imgrc=egXI6klyEXzr4M%3A&usg=__xwSZBAdxFiy1pfFmFNsdjb326aw%3D

[Amadis of Gaul]

Holy IDG! That's some serious smarticle stuff there!

[Checks Complete]

Hi Mstram

As a retired Directional Consultant I could not resist this thread. Many of these navigational 'Rules of Thumb' are a guide and work reasonably well in the local area but once you look at long distances there are too many variables for them to work.

The chances are the Garmin Simulator showed a Great Circle track modified for the local Magnetic Variation (Dip) at the start point. The Great Circle track is constantly changing so is the Magnetic Variation. Even if you had put in the destination as 90N, 00E/W the True Track would be a constant 360 True but the Mag Track will change as the Magnetic Variation changes. The rate of change is not constant. See the link on Goldenrivett's post.

If you really have difficulty sleeping at night, look up steering with reference to Grid North!

Kind regards

Checks Complete

[Amadis of Gaul]

Hi Mstram

As a retired Directional Consultant I could not resist this thread.

A few questions:

1. What is a Directional Consultant?
2. How does one get that job?
3. How do the responsibilities of a Directional Consultant differ from those of a Non-Directional (Omnidirectional?) one?

[m39462]

Gents, all this talk of approximations and Great Circles would be meaningful in many scenarios, but the poster's confusion is much more mundane: Had he targeted
the correct location for the magnetic north pole, his DTK would have been 0 degrees magnetic give or take, and would not have changed no matter how far west he went, or in fact anywhere on Earth he went.

[Goldenrivett]

Hi m39462,
all this talk of approximations and Great Circles ... his DTK would have been 0 degrees magnetic give or take, and would not have changed no matter how far west he went, or in fact anywhere on Earth he went.

His GPS would calculate a great circle track and apply the local Magnetic variation (which depends on his position on the Earth.)
Since the True North Pole and Magnetic North Pole are not coincident, then his GPS Initial Great Circle Track would change depending on how far East or West he went along 0 degrees Lat.

He could eventually find the Magnetic North Pole if he tracked North using his compass, but his journey would certainly not be along a great circle.

[m39462]

Goldenrivett, I'm not sure if you misread the original post or are being uberprecise. mstram's DTO was to where he thought the magnetic pole is, and that largely means that the true pole and magnetic variation don't enter into it. But I made an allowance for uberprecision too when I said "0 degrees give or take", as the nonuniformity of the earth's magnetic field will likely cause the plotted course to vary from exactly 0 degrees magnetic.

Or to put it another way, the GPS will plot a great circle track to the magnetic pole. If the magnetic field were uniform then its true bearing would be exactly the negative of the variation no matter where he started, and converting it to magnetic would yield exactly zero. But since the field is not uniform his result would be zero, give or take.

However you think about it, the only reason he saw any significant drift from zero as he sallied westward was that his plotted destination was several hundred miles away from the actual magnetic pole.

[Goldenrivett]

m39462,
the only reason he saw any significant drift from zero as he sallied westward was that his plotted destination was several hundred miles away from the actual magnetic pole.

If you read Checks Complete's explanation more carefully, his statement that the 'Rules of Thumb' are a guide and work reasonably well in the local area but once you look at long distances there are too many variables for them to work." is correct.

The "rule of 60" which the OP applied is approximate and only works on a 2 dimensional flat surface. With large distances (e.g. over 5,000 miles) spherical trig is necessary as the curvature of the earth's surface is no where near a flat plane. Even if the correct position of the Magnetic Pole had been entered correctly, "the rule of 60" would still not produce the correct result.

[m39462]

Absolutely correct that spherical geometry can only be approximated by plane geometry in the small, but that is not at work here. Given the experiment that the OP tried to do, had he correctly situated the magnetic pole he would have seen a constant 0 magnetic track from any point even if the earth were flat.

EDIT: And, for good measure, if he moved his experiment from the equator to a point only 100 miles from the magnetic pole he would again see the same constant zero track, on a spherical earth or a flat one.

[Checks Complete]

Directional Consultant is the nickname sometimes given to Flight Navigators. As a member of the Flight Deck Crew we were often the subject of jokes from the rest of the crew particularly from the 2 Winged Master Race (Pilots). However, in the days before GPS and INS, it was amazing how the crew's attitude changed on long Ocean crossings and operating close to the North Magnetic Pole. Gyro steering is not the easiest concept to get your head round! Close to the Magnetic Poles (100nms or less) a direct reading Magnetic Compass is very inaccurate.

To go back to the original post the reason I mentioned Great Circle Tracks is that a VOR Radial is a Great Circle adjusted for Magnetic Variation at the beacon. If one looks on an Airways Chart where the Airways are based on VOR Beacons. Where the VOR beacons a significant distance apart the inbound and outbound radials may not be 180 degrees apart. This is because of changes of Great Circle track and Magnetic Variation.

Kind regards

Checks Complete

[oggers]

mstram:

I'm trying to relate the VOR radial "rule of 60" to magnetic compass headings...With the "rule of 60", 5544 / 60 = 92. Therefore after flying ~92 nm west, the DTK should change from 353M to 354M.

Over 1 degree the '1:60 rule' is actually 1:57. Therefore a more accurate distance for a 1 deg DTK change in your example is 97NM. Also 90 deg to the initial DTK is 263 as opposed to the 270 mag you used. When I put these figures into my garmin as a flight plan the change in DTK is indeed 1 deg, so the basic trig works well enough. [On second thoughts and bearing in mind that 60NM is 1 deg of lat, I guess that is an odd result. Hmm..]


Since the field is all over the world, I think the north attraction just gets stronger right at the pole


The needle aligns with the magnetic field. The field is not stronger at the poles. It is stronger near the centre of the earth. If you look at a diagram of the magnetic field you will see that the lines of flux are parallel to the earth's surface at the equator but nearing the poles they start to 'dip' towards the surface. Therefore if you stood directly over the north magnetic pole and put a compass on its side the needle would align itself by pointing at your feet, but with the compass held horizontal it would be unable to properly align itself with the magnetic field. Hence the utility of a magnetic compass is degraded near the poles where the field is more vertical than horizontal.

[KenV]

I'm trying to relate the VOR radial "rule of 60" to magnetic compass headings.

I *think* the thing about the operation of the compass, is though the Earth has a magnetic "north pole", the magnetic field is obviously not just concentrated on that spot. (Like a theoretical VERY long range VOR would be).There seems to be some confusion about the "rule of sixty". That rule states that for each 1 degree of error in your heading, you will accumulate (about) 1 mile of error in your position for each 60 miles flown. This is entirely a geometric problem totally unrelated to magnetic vs true north. Further, because a nautical mile is about 6000 feet, the 60:1 rule also applies. Thus, each one degree error in heading will result in 100 feet error in position for each NM flown. Keep in mind these are only approximations and only valid for small angles.

This also works great for flight path vertical angles. Each degree of climb (or descent) gradient will result in 100 feet increase (or decrease) in altitude for each NM flown.

Or did I misunderstand the question?

BTW, there is an error in the following statement: "Since the field is all over the world, I think the north attraction just gets stronger right at the pole." Actually, no. The magnetic poles generate lines of magnetic field. A magnetic compass aligns with the magnetic lines and point to the magnetic poles. But near the poles, the magnetic lines are vertical or near vertical, so a magnetic compass is very erratic near the poles. At the magnetic poles this is like the "cone of confusion" right above a VOR beacon.