my phone compass point in a different direction,why?

[im_Siana]

I'm new to cycling and have been trying to rely less on turn-by-turn navigation so I can get better at following directions on my own. I have a small compass mounted on my handlebars, and I also check the compass on my phone every now and then. Most of the time I don't pay much attention if they're a few degrees apart, but I've noticed something that keeps happening.

Whenever I stop for a couple of minutes, like at a crossing, a café, or just to take a drink, my phone compass sometimes points in a noticeably different direction from the handlebar compass. If I walk a few steps with the bike or start riding again, the phone heading slowly changes and sometimes ends up matching the bike compass much better. Other times they're still slightly different.

I've already tried calibrating the phone compass with the usual figure-eight movement, but it doesn't seem to completely solve it. The phone is mounted on the handlebars next to my front light, while the bike has a metal bell, phone mount, cables, and a small handlebar bag. I don't know if any of those things can interfere with the compass reading or if what I'm seeing is just normal behavior.

I also checked an online compass in my phone's browser to compare the readings. As far as I understand, an online compass uses the phone's GPS location together with its built-in sensors to display the current heading. One of the sites described itself as a digital direction finder, which seemed like a useful way to double-check the direction without installing another app, but it showed almost the same heading as my phone instead of matching the compass on my bike.

Is it normal for a handlebar compass and a phone compass to disagree after stopping for a few minutes, and if so, which one should I trust when I'm trying to figure out which direction I'm actually facing?

 

[rogerzilla]

What's your bike made from?

 

[ebikeerwidnes]

The phone is working it out from GPS
and it only gets the actual location from that - so it takes several GPS locations in a short period fo time and works out th direction you are going in from that
So if you stay static then it cannot work it out - and just looks at the last few and makes an assessment from that
so it you go into a cafe and park your bike round the corner then it gets confused sometimes


The other possibility is that the magnetic compass is getting confused by the iron in the bell and elsewhere - including any electric circuits neaby
But that is less likely as if that is the case then it will always be out a bit and never agree

 

[C R]

The phone is working it out from GPS
and it only gets the actual location from that - so it takes several GPS locations in a short period fo time and works out th direction you are going in from that
So if you stay static then it cannot work it out - and just looks at the last few and makes an assessment from that
so it you go into a cafe and park your bike round the corner then it gets confused sometimes


The other possibility is that the magnetic compass is getting confused by the iron in the bell and elsewhere - including any electric circuits neaby
But that is less likely as if that is the case then it will always be out a bit and never agree

Some (most?) I think have magnetic sensors, which they use to simulate a compass, so would be just as affected by the same things that would affect a magnetised needle.

 

[ColinJ]

Some map software has an option to choose between...

1. North at top of screen
2. Travel towards top of screen

If yours is set to option 2 and your GPS position jitters about (which it probably will) then the screen map might be auto-rotating to adapt to that. The other thing that I have experienced is delays in auto-rotation so it can carry on for a few seconds after turning.

 

[presta]

Some (most?) I think have magnetic sensors

What about phone cases like mine that have a magnetic clasp?

 

[C R]

What about phone cases like mine that have a magnetic clasp?

I guess in that case the magnetometer won't work.

 

[a.twiddler]

Interestingly (to me at least) my Garmin etrex 20 has a compass setting, though I haven't used it a lot. It's probably not very sophisticated, and as suggested up thread it's one of those systems that needs some movement before it can detect a satellite and give a compass heading. More up market systems can probably give a static reading. I really appreciated it a week or so ago after getting off the route of the Wirral Circular Trail and set a course that eventually intercepted the trail again so I could continue on my route.

I suspect whatever compass you have will not be 100% accurate. If you've only got one, you probably don't worry about it, as it's accurate enough. If you've got a standby compass, that's when you start to worry, as they'll probably disagree to some degree.

 

[ebikeerwidnes]

Some (most?) I think have magnetic sensors, which they use to simulate a compass, so would be just as affected by the same things that would affect a magnetised needle.

My phone 2 ago was like that

but the current one - and the one that is presumably still by the side of a track ona n island in the outer Hebrides - clearly do not
Basically they just get confused if you don;t move

 

[Bristolian]

As stated by @ebikeerwidnes, the compass on your phone isn't really a compass. It uses GPS to calculate your position and, when moving, the change in position to calculate your direction of travel which is transposed into a bearing for display. When stationary it knows your position but can't tell which way you're facing. Most phone compasses will change heading quite wildly - maybe up to 30 degrees - after a few moments of being stationary and take a second or two to correct itself when you begin to move again.

The handle bar compass will be influenced by anything nearby that contains Iron or any wires with a current running through them (such as your front light) or any electronic equipment that doesn't have sufficient shielding.

Pull up a sandbag and I'll tell you a war story. Back in the day, when I was an Army apprentice, we did a practical experiment to see how far you need to be away from a Chieftain main battle tank for a Sylva magnetic compass to not be affected by the vehicle. 52 metres to the sides and rear or 54metres to the front, if you're interested.

We also had to calibrate a device mounted to the rear of a Land Rover that gave its position as an 8-figure grid reference. This piece of equipment (the Sperry Chobham Navigator) relied on two coils, mounted at 90 degrees to each other, on an aluminium beam stuck out of the rear of the Landie. To calibrate it you first had to enter the grid reference of your position and then drive due east (or west) for a couple of miles at about 30mph, then turn around and return to the start point. The coils would measure the earth's magnetic field and use that to calculate the grid reference of the vehicle anywhere in Europe. We used the A329 to the east of Bracknell as it runs almost due east-west for about 3km. I hasten to add that was the one and only time I saw that equipment in my 29 years of service (thankfully)