Compasses in the store never agree with each other...?

I’ve been looking for a compass to keep in my car to help me keep my bearings, and it seems like I run into the same phenomenon in every store I visit; none of the compasses seem to agree. I’ve tried holding them at equal heights, making sure that they’re level, etc., and I still get a pretty wide variety of readings. Is this simply a case of the magnets inside the compasses influencing the nearby compasses to give false readings? Should I just go ahead and grab any of them instead of looking for one to give a correct reading right away?

Compasses are affected by any nearby ferris metals, it’s possible the compasses are all affecting each other, also possible that there are other objects in the store which are affecting the compasses.

The compass will also be affected by your car. Both by the movement of the car and any ferris metals in the car. Compasses used in aircraft generally have a deviation card which is a list of bearings to steer to get the one you want. Eg for 090 you may need to head 087, not to be confused with magnetic variation which is an entirely different matter.

All the car compasses I’ve seen have calibration screws to align them properly. Since they’re intended to be used around significant amounts of steel, which will affect their operation, they’re made to be adjusted to the particular environment in which they’re installed.

The calibration process entails orienting the car alternately in known north/south and east/west directions, and using the screws to align the indicator properly. Once this is done, the compass should be accurate so long as its mounting point inside the car isn’t changed.

Then just remember not to lay a big hunk of steel next to the compass while reading it.

Why not buy a GPS and really keep your bearings?

ummm, it’s Ferrous!

anyone, anyone, Bueller? Bueller?

Try taking them outside & spin them around a couple of times & see if they agree. There are digital compasses for the car that orient themselves to a satellite. They aren’t GPS units, I did see one for sale but I didn’t buy it. Might be some on search there for ‘compass’

Thanks. I think I’ll look into one of the digital compasses that orient themselves by satellite, then.

I’m not in the market for a GPS right now because I can buy an atlas and a compass at a fraction of the cost.

Or at least reasonably so. Ships compasses have very sophisticated arrangements with strategically placed magnets to adjust them to counteract the effects of the ship’s fabric. However, even then they are not one hundred percent accurate in every direction. I doubt that something you would buy for your car would be exceedingly accurate at every point of the compass.

There are professional compass adjusters who are employed by ships to adjust compasses.

I would think that the precision alignment of a compass is a bit more crucial for ships than for cars, since an inaccuracy in a ship’s compass could result in major errors in navigation, while being a few degrees off in a car won’t make that much of a difference as long as the driver could tell that he or she was pointed roughly northward…

Agreed. Although modern ships rely largely on gyro compasses, radar and GPS for navigation.

I think those are just magnetic field sensors with digital displays. They should be affected by nearby metal the same way compasses are. I don’t know of any satellite-based compasses that don’t use GPS signals.

On one research balloon we flew, we used magnetic sensors to measure the orientation of the gondola. It’s extremely tricky to do accurately. Nearby electrical circuits and pieces of metal have big effects, and the two onboard sensors would rarely agree to within 2 degrees. (Well OK, that’s as accurate as we needed so we didn’t try much harder)