my phone compass point in a different direction,why?

[presta]

I lost an atlas page from the bike once, when the vacuum at the back of a passing lorry popped the map holder open.

 

[Bristolian]

Thanks @ebikeerwidnes and @Bristolian I've often wondered why my phone compass is so poor.

Given all the potential interference from cycle equipment is it worth having a compass mounted on one's bike? I'm just interested, not planning to use one.

Is it necessary to spend a significant amount to get a compass that won't suffer from interference?

TBH, I wouldn't expect too much interference from your bike itself unless the frame is steel.

A compass that was completely immune to interference would be self-defeating - it wouldn't work if you block out all the magnetic fluxes around it. The Army used to fit a quasi-compass to a small number of main battle tanks back in the 1970's but at over £10,000 each back then they were treated with kid gloves and hardly ever used. The only place I saw them used was in BATUS - a 2,500 square mile training area in Canada that was just rolling prairie with virtually no features that could be used for map reading. They were installed in each troop leaders' tank so 1 in 4 vehicles.