[QUOTE=Bosda Di’Chi of Tricor]
Coming from somebody who makes maps for a living.
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Maps or photographs of maps?
http://boards.straightdope.com/sdmb/showpost.php?p=8821125&postcount=11
[QUOTE=Long Time Lurker]
Maps or photographs of maps?
http://boards.straightdope.com/sdmb/showpost.php?p=8821125&postcount=11
[/QUOTE]
Maps are made by means of Aerial Photography. I have been an Aerial Photo Tech with the Tennessee dept Of Transportation for years now.
There’s a great big lump of steel upfront in the shape of the engine. Absent of correction, the compass needle will simply point to that (or at least to the extent of making bearings very unreliable). Some car compasses come with compensating magnets which you screw in and out to balance it out (cheap ones don’t). To do this you need to align the car on North first, selecting from the map a road which just happens to be aligned on true North, then swing it through the three other cardinal points of the compass, making further corrections as you go, just as they do (or did) with airplanes in WW2. This is probably too much trouble for most people to do, even if they realise the need for it.
It’s actually a bit worse than that. Modern cars have such high electromagnetic fields, than when you turn the key on, all of your calibrations go straight out the window. :smack:
[QUOTE=GorillaMan]
Trouble with using a compass in Wiltshire is all the ley lines messing it up.
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All you need to do is pass your hand over the compass, palm up, slowly three times while incanting “Ley go, ley go, ley go”.
Well anyway, I’ve never had a problem with ley lines and compasses in Wiltshire after doing this, so…
[QUOTE=mnemosyne]
A compass would be next to useless around Montreal… the highways here are about as far off from geographic direction as I’ve ever known!
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My truck has one - I rarely look at it. Try the Bay Area for odd roads.
"I’m on 101 north, going due West. Perfect.
My compass really helped me out earlier. Got lost south of here, but I know the roads north of here really good so I just kept following my compass north till I hit a familiar road.
I might still be driving around Connecticut without that puppy.
My guess is consumer demand hasn’t been that high for them.
[QUOTE=Voyager]
My truck has one - I rarely look at it. Try the Bay Area for odd roads.
"I’m on 101 north, going due West. Perfect.
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Near me you can be traveling on the I405 North make a 90 degree transition to the US 101 North. (360-90=360)
Or if you are going in another direction you can go from the US 101 South to the I 405 North by making a 90 degree turn. (180+90=360)
WTF?
They did not teach me this shit in high school geometry.
The compass on the mirror in my GMC pickup truck works correctly and I am sure it is not bothered by the metal in the body, the engine block, or the wiring and computer system.
As for being useful, I don’t think it is good for much.
[QUOTE=papaw]
The compass on the mirror in my GMC pickup truck works correctly and I am sure it is not bothered by the metal in the body, the engine block, or the wiring and computer system.
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Of course it does, it was designed by the same people that designed the truck. They know what the issues are, and can design around them.
the two guys standing in a quonset hut in China on the other hand…
Eh, until I moved to the US I’d never used “official” cardinal points except to orient maps.
The cardinal points in the town where my mother and brothers live are Barrio, Monte Cierzo/El Cristo, Pamplona and Zaragoza.
The cardinal points in Barcelona are Right, Left, Mountain and Sea.
A couple weeks back I was driving around Zaragoza, with my mother (who hasn’t lived there since she was 11 but still thinks she knows the place) uneffectually trying to give me directions. She kept using Up and Down but I had no idea where those are in Zaragoza! (Apparently Up means “in the direction of the Basilica del Pilar”… which is at the river so sort of the opposite of my notion of up).
So at least in Spain, a compass in a car would be useless unless it was able to show local cardinal directions 
The compasses installed in cars are calibrated for the car. They can lose the calibration, in which case you have to drive the car around in circles for it to recalibrate.
The OP might want to look into a cheaper hand-held GPS. Although it will not talk to you, it will give the correct compass heading regardless of magnetic field.
I used to work for a place that did deliveries to people’s homes and I can say from experience that 90%+ of the customers I dealt with had no clue how to get around via compass directions anyway.
Customer: “Ok, so head down Vine about three blocks, then turn left on Main”
Me: “Turn north onto Main…”
Customer: “No, left onto Main.”
I doubt many people would really have a use for the things even if they were available.
[QUOTE=Rick]
Near me you can be traveling on the I405 North make a 90 degree transition to the US 101 North. (360-90=360)
Or if you are going in another direction you can go from the US 101 South to the I 405 North by making a 90 degree turn. (180+90=360)
WTF?
They did not teach me this shit in high school geometry.
[/QUOTE]
Actually, in the Bay Area, you can find yourself traveling north on 580 west and 80 east simultaneously. Although, why people get perturbed because roads aren’t going their labeled compass direction at some point, I’m not sure. Unless the road travels in a straight line, it can’t maintain a compass heading, and I’ve never lived where roads usually did that, or even came close.
[QUOTE=silenus]
Why? It’s not like you can suddenly decide to take your Camry North by Northwest. You have to stay on the roads, which seriously limits the usefulness of a compass. On top of that, imagine all the Little Old Ladies as they toodle along at 55mph on the 101North. They glance at the compass, and it says they’re heading West. Oh, noes! Much panic among the bluehairs, and traffic comes to a complete stop. Thank you, no. Leave the compasses to sailors and hikers.
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You may not be able to lay in a course for NNW, but at least a compass would reassure you that you are travelling in the right direction more or less, or not. However, car compasses have a reputation similar to cruise control, i.e. something you’d only use if you are elderly or anal (and no association between the two is hereby intended). I think this is a little unfair for what is basically a very practical feature.
[QUOTE=yabob]
Actually, in the Bay Area, you can find yourself traveling north on 580 west and 80 east simultaneously.
[/QUOTE]
Hey, I was gonna say that! Anyway, here’s a map of the area.
You guys have missed the best example of fucked up directions…
In southwest Virginia, you can be on I-77 South and I-81 North at the same time, and while doing so would be heading almost due East the whole time.
The signage looks like this:
Gotta cut and paste the link… But it’s worth it, the signs are pretty messed up. =)
If your compass is not working correctly on your car, it may need to be recalibrated. Try driving your car in as tight a circle as possible making about three complete circles. That should do it.
We have to reset ours after driving on some of the twisting and turning backroads up around Monteagle, Tennessee.
Capt. Cindy is not amused and will be looking for your car on the freeway now that she has retired from the navy. Fortunately for you, she is leaving for a trek to Nepal next week…
[QUOTE=Zoe]
If your compass is not working correctly on your car, it may need to be recalibrated. Try driving your car in as tight a circle as possible making about three complete circles. That should do it.
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RTFM. This is not a universal procedure.
[QUOTE=Rick]
Of course it does, it was designed by the same people that designed the truck. They know what the issues are, and can design around them.
the two guys standing in a quonset hut in China on the other hand…
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Absolutely. I tired one of those ball-in-water compasses and it immediately showed me how much of a pain the magnetic field of my car is (2003 Ford Focus). While parked I could make it point in any direction I wanted by sticking it to a different position on the windshield/dashboard.