Emergency compass

[bobmrg]

It might be old news to most of you, but it is news to me. The iPhone default compass app indicates direction relative to true north. I searched for iphone + compass and found a free app that lets you choose between true and magnetic. Display is like a pocket compass from back in the day, but it sure doesn't jiggle around as much as a wet compass.

Bob Gardner

 

[Jim Logajan]

Where are you going to hold the iPhone while using it as a compass? How will you know how much local disruption is impacting its compass? Lay it on a kneeboard with a steel spring clip or other magnetic material and it will be GIGO.

No thanks, that is pretty far down on my list of backup devices.

Helps to actually try it before dismissing it:

I've had no problem using an iPhone's compass in an airplane - or anywhere in my home - or in my car. The difference between the whiskey compass was not large - certainly less than the amount of wiggle you observe in a whiskey compass in mildly turbulent air.

By the way - the chip that measures the ambient magnetic field vector in an iPhone or iPad (and probably all mobile devices using that or similar chips) yields a 3 dimensional vector. As a result, on the iPhone and iPad at least, they've programmed it so that the main relative axis (for purposes of display) points to the top or back of the device, so you can hold the iPhone upright or even upside down and see which way the device's back or top is facing. Flip it upside down and see what happens.

 

[Jim Logajan]

Not all those tablet magnetometers work worth a damn, especially around electronic equipment (sometimes, it would seem, even the tablet itself). Never bet your life on untested equipment, or you may lose the bet.

Magnetic field distortion should affect a traditional compass in the same way that the magnetometers in a tablet should be affected. Unless you think they are somehow different in that the measure different forms of magnetic fields!?

I think that the same caveats apply to use of any compass - keep it as far as possible from ferrous or other metal alloys that may distort magnetic fields. The one nice advantage of, say, an iPhone is that you can hold it vertically to get a steady reading - something you can't do with a traditional compass. Obviously the latter needs no power.

As to untested equipment - it should not take that long to check the operation of a compass in a cockpit or on the hiking trail. I found it works best to simply hold an iPhone in my hand maybe a foot or two away.

 

[Jim Logajan]

Flux gate magnetometers are subject to EMI that permanent magnets are not.

You don't even know what kind of sensor is being used in mobile devices.