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Old 09-02-2011, 00:15   #1
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Variation Getting More Variable

Now I realize how out of fashion the lowly and ancient magnetic compass is in light of the New Dispensation of GPS, but I still like to call myself Hornblower and refer to the little Plastimo compass in the cockpit. In fact, I use it to, you know, navigate.

On my local chart, the compass rose says, as they do, something along the lines of "10 degree W deviation (1994)". That means if I read 90 degrees on the compass, it's actually 80 degrees T. It also suggests that the last time someone was around here with a Great Big Azimuth Compass was going on 20 years ago.

Unfortunately, the stately and predictable movements of our Magnetic North Pole are becoming...well, skittish, and the rate of deviation (also marked on most charts) is probably undervalued.

Does this affect the average sailor? No. When it hits 11 deg. W in my home waters is unlikely to affect my helming or pilotage as I can't steer better than a 5 deg. wobble most of the time. Nor would I want to.

But I have found it interesting that there are starting to be real-world effects due to the Wandering Pole:

Tampa airport runways renumbered due to magnetic north movement

"The primary runway at the airport is designated 18R/36L, which means the runway is aligned along 180 degrees from north (that is, due south) when approached from the north and 360 degrees from north when approached from the south. Now the Federal Aviation Administration (FAA) has requested the designation be changed to 19R/1L to account for the movement of the magnetic north pole."

I point this out as a navigational curiosity and as a small token that the Earth doesn't really care what is printed on our charts.
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Old 09-02-2011, 00:39   #2
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Don't worry! It'll come back. Just like the snow every 3-5 hundred years.
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Old 09-02-2011, 09:40   #3
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Trust me, I'm not worried. But I do find that the pace of variation has dramatically accelerated to indicate we only have a partial clue on how these things work.
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Old 09-02-2011, 10:31   #4
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Apparently North Pole is heading t'wards Siberia and weakening.
I have actually noticed because magnetic bearings applied to old charts have noticeably shifted since I started sailing.

... the now-serious theory is that we could be about to have a pole shift.
When North Goes South: The Nature of Things with David Suzuki : CBC-TV
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Old 09-02-2011, 10:36   #5
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So why do airports use magnetic designation instead of true?
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Old 09-02-2011, 10:42   #6
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So why do airports use magnetic designation instead of true?

Airplanes must carry a magnetic compass, but not GPS.
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Old 09-02-2011, 10:55   #7
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<RANT>This thread is heavy with the myth that GPS on a small craft somehow reports heading.</RANT>
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Old 09-02-2011, 11:10   #8
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No, it isn't. It's about compass variation. GPS rants are elsewhere.
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Old 09-02-2011, 11:35   #9
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Quote:
Originally Posted by harmonytek View Post
So why do airports use magnetic designation instead of true?

Airplanes must carry a magnetic compass, but not GPS.
Yea, but ships carry magnetic compasses but ranges are given in true degrees. It must be tradition.
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Old 09-02-2011, 11:46   #10
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Asked a pilot friend this once, and his answer was that in the early days of airport traffic control, small aircraft flew VFR using magnetic compasses. Designating runways by radial degrees made for an easy visualization for approach, before the days of radar vectoring.
Any active pilots on the board?
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Old 09-02-2011, 11:49   #11
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It's good to have a magnetic compass if you want to get to Siberia. Just follow the li'l arrow.
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Old 09-02-2011, 12:06   #12
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YEP. In some cases new data has to be applied to an older chart - the correction info in the mag rose no longer binding. I guess when they made the chart they never thought such a thing as a 'budget cruiser' would ever appear on the surface of the Earth ;-)

They were all working, not messing around.

PS Actual data is available though from various sources.

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Old 09-02-2011, 12:23   #13
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If you need to update the rose go here; Magnetic Declination (Variation) Home
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Old 09-02-2011, 12:27   #14
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Quote:
Yea, but ships carry magnetic compasses but ranges are given in true degrees. It must be tradition.
I believe most ships actually use a gyro compass and only have magnetic as a back up. As to aircraft runway numbering, the system was put in place when most flying was done under visual flight rules or VFR, before the days of ground control, IFR and CVFR. Numbering was not exact and usually rounded to the nearest 10 degrees. In most areas this is referencing magnetic north, but in Canada's NDA runways reference true north because of the big declination that close to the pole.
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Old 09-02-2011, 12:35   #15
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Most ships do use gyro's with a known gyro error which can be calculated using the bridge wing gyro repeater by doing an azimuth of the sun or other celestial body. This provides a heading correction.

Two GPS antennas can also used to calculate gyro or magnetic steering compass error, should this GPS based heading system fail.

One example of a satellite compass...
http://www.furunousa.com/products/Pr...x?product=SC30

One degree is the smallest increment that recreational boaters need to worry about. If a chart variation is wrong by more than one degree then it might matter...depending.

Where does it say the amount of annual change has increased? I could not find anything on this.
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