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compass and equator
i'm riding with a mate to namibia this summer and we're taking a gps. i want to take a compass as a backup. but i'ff always heard storries that when you cross the equator with a compas mend for use in the northen heminisfere that when comming close to or crosing the equator is will become stuck because the needle will "lift" its point and will try to point trough the glass. i understand the idea and it sounds plausible but does this actually happen? or is this just a tall tall. since there are "world compassas" avalible with should not have this problem it sounds plausible but then again, companies will try to have us believe any thing if they think they can make a extra buck. so do i need to buy a expensive "world" type compas or is my old dutch recta good enough?
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My opinion
I think it's because the needle points to the magnetic pole of the Earth (North) so when you are at the equator the North Pole is actually at a 45 degrees direction "down" rather than in a straight line in front of you (or behind you) hence the needle "dives" getting stuck: it is pointing underground to the N Pole, try to tilt the compass a bit.
I'd expect good compasses have the spindle with enough clearance to minimise this. That's my opinion but I may be wrong. |
Courtesy of wikipedia.. Compass - Wikipedia, the free encyclopedia
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speachless....
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I thought you might like it.. It's good to know you're right! :)
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I suppose the crappy ones that come in a glass or plastic ball wouldn't be affected or at least they wouldn't get stuck but they are usually cheap therefore not very precise I would think.
http://www.magnifier.com.tw/proimages/w92.jpg |
I had one like that, but it got destroyed when I put it in my (magnetically secured) tank bag.. d'oh!
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This one was ok
I have a suunto diving compass like the one below, bought in london. I used it to dive in South Africa and was working fine there. You could see the needle dip a bit, but because of the design if the compass itself it didnt get stuck. (FYI - its a SK7 model)
http://www.suunto.com/media/suunto/i...large_fbb3.jpg |
thanks mate, thats the kind of info i'm looking for, i know the theorie but want to know if the influence is actualy so great that my compass won't work.
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But if it 'dips' at the equator, what does do when you are in Australia? turn upside down?
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Bill, Refer to post number 3 from Phoenix; the compasses can handle it, bearing in mind that they are not particularly accurate anyway, pointing toward the magnetic pole rather than the true north/south. The dip angle is greatest at the equator I believe, based on a more or less symmetrical magnetic field around the whole globe (perhaps a representation of that field should be on the HU logo?!) A bigger problem would be if you got close to the magnetic pole while using a compass because then the compass would be showing you that way rather than toward the true north i.e. the magnetic deviation (I mean variation, as per Smitty's post) increases. |
Don't worry about it!
As the compass points to "Magnetic North" with the pole position at approximately 77 degrees North, 122 degrees West (in northern Canada) there is a correctable offset to "True North". The biggest problem with a magnetic compass overland is knowing the magnetic variation for your locale. You can buy or download a "Pilot Chart" which give the lines of magnetic variation on the earth for your locale. If you are in an area where there is no variation, then the magnetic compass and "true" are closely the same. Some areas of the world have magnetic variations in the range of 25 degrees(east or west) which must be applied to the compass. That means if you are traveling southeast (135 degrees), and the local variation is 25 east, then the compass heading you must maintain is 110 degrees! Your GPS will track 135 degrees (based on true polar north). The other influence on the magnetic compass is magnetic deviation which is based on the magnetic effect of your bike influencing the compass. It is minor and would be difficult to compute. Lastly, if you are in an area of "Magnetic Anomanolies" (lots of Iron in the ground) (they are listed) then your compass can go haywire and point to the anomanalie. Here is an easy way to correct the variation (if known) and go in the correct direction with a magnetic compass. TVMDC-AW.
That stands for True Virgins Make Dull Companions--- At Weddings. Under the T put the "True" direction you want to go... 135 degrees. Under the V put your magnetic variation and label it east or west. D is for Deviation which we will disregard. Under C will be your magnetic compass heading in relation to true. AW means "Add West". So if you have 23 degrees west variation, you add it to 135 and get 158 degrees. If it is easterly, you subtract and and you C would be 110 degrees. C is the magnetic compass heading you will follow.Regarding the dip, better compasses are corrected with with flinders bars and correcting magnets. It's the variation that can really throw you off track.... Good luck and don't get lost. Best regards, Smitty |
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Way to go Smitty! UK maps and I guess most maps also show the variation information for Grid North which is related to the differences caused by a flat map trying to represent a curved, more or less round, globe. So, we end up being able to deal with True, Magnetic and Gird North in order to travel. |
Given that the earths magnetic poles flip polarity on average every 300,000 years, then we're approximately 500,000 years overdue for a flip! :)
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Quick compass check using the sun
At sunrise or sunset, just as the sun is breaking the horizon, line up the compass as best you can and check the heading. As the sun "rises in the east and sets in the west, the compass heading should be near 90 degrees in the morning and near 180 degrees at sunset. Time of year and latitude will effect the results as the sun travels 23.5 degrees north of the equator in summer and 23.5 degrees south in the winter. You can tell which side of the sun you are on (bearing north or south of you just by looking). At "noon" when it is overhead, Is it directly overhead or more to the north or south of you. That is the suns bearing in relation to you (at your lattitude). The more directly overhead it is, the more accurate you morning or evening compass check will be. If you find that your morning check (you want to see near 90 degrees) is say 115 degrees, then, using the formula TVMDC--AW you can backtrack and get a rough idea of the variation in the area. People have sucessfully traveled the planet for hundreds of years by magnetic compasses. They do work. I'll bet alexpezzi's key chain compass was better than what Magellan used and he didn't do to bad. If all else fails a good pocket compass is a good investment if you know how to use it. Best, Smitty
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Magnetic tank bag
If you are using a mounted magnetic compass and use a magnetic tank bag, then you are going to have issues with deviation. The tank bag will really screw up the compass and you will you will never achieve accuracy with the compass. Even by walking away from the bike with the compass, the sphere of influence could not be determined. Use a "Strap On". Thank's Phoenix. Never considered the bag. Smitty
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good tip, thanks mate
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It's worse than that.. I don't put any electronics in the tank bag... strong magnetic fields (those little magnets are realy strong) can play havoc with electronics..
I found that my compass was re-magnetised by the tank bag, so I couldn't even use it once I'd walked away from the bike.. I had to throw it away. Next time, I'll go for a strap secured tank bag.. definitely not magnet secured! |
What goes around, comes around
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