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Compass wrong on rear view mirror!?


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Wierd problem but I happen to have a Magellan Road Mate 700 navigation unit and noticed the the compasses (nav. unit vs. mirror- 01 OBW-LL Bean) don't agree. When I was playing with the mirror control I noticed that when you keep the button depressed there is a "zone" option that you can change numbered 1 through 15 (I think). I assume the zone means what part of the Earth you are on.(?)

Anybody had this problem?

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For the rear view, set the appropriate zone that you're in (should be in your owner's manual) and the error should go away. Note that if you drive to another zone, you need to set the new zone in. The Magellan GPS is corrected for whatever zone you're in...so I would first trust the Magellan, then verify by looking at your rear view compass (if it is set for the right zone.) Also, the rear view compass can be off for a variety of reasons...like running electronic equipment too close to it, cell phone towers, all sorts of stuff that can make it unreliable.

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Description of Magnetic Variation on Wikipedia.

 

Rough Visualization.

 

Now if I remember one of my flight instructor's side comments correctly, the frame of the car, electricity flowing through nearby wires, or anything else that can generate or alter a magnetic field can interfere with the compass. So if the compass seems really off, I'd start searching for interference.

 

I want to say my flight instructor said the compass had to be adjusted when it was installed in the aircraft, adjusting for any interference the other equipment/design of the aircraft. I doubt it could've been more than a few degrees, so I don't think you're gonna notice/complain about it in the car. A few thousand feet up, though...

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It funny to see the magnetic compass in the cockpit spin around and tumble when you power up the generators and avionics. And in the worst case scenario, I'm supposed to rely on this compass when all the redundant systems fail? Now I know why early navigators used the stars, sun, time, calendar and moon first then used their compass.

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It funny to see the magnetic compass in the cockpit spin around and tumble when you power up the generators and avionics. And in the worst case scenario, I'm supposed to rely on this compass when all the redundant systems fail? Now I know why early navigators used the stars, sun, time, calendar and moon first then used their compass.
I thought you used Interstates....

 

In Germany our chopper pilots could always find home because of the immense nuclear plant cooling tower nearby. In the day only medevac choppers had GPS. Now LPC's have GPS.....

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My compass/rear-vew-mirror on my 2002 WRX went south on me and I had it replaced under warranty from my dealer. He told me that Subaru received a bad batch of mirrors and were replacing a bunch free of charge. You should check with your dealer.

 

GPS is worth every penny. I highly recommend anything made by Garmin. Just remember that those DOD satellites were originally used for targeting nukes so they're pretty reliable. Nothing like triangulating a fix from 3 satellites to figure out where you are and where you're supposed to go...

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  • 7 years later...

There is a calibration procedure that you should do periodically - check your owners' manual.  It corrects for changes in the magnetization of the body so you can get true readings.  Basically you push a button sequence to put it in calibration mode, then drive in a circle in an open area free of nearby steel objects.

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