I often hear runners entering long trail races that they are going to use electronic equipment such as a GPS tracker or an IPhone as a compass. Whilst the puritan in me bulks at this idea preferring the old fashioned compass I am a technophile who is happy to use GPS equipment when necessary but be warned!
Ask the majority of users how an IPhone finds you they will tell you it is a clever satellite that can find you and there for work out your position on the Earth's surface but they do not realise it is the device itself that uses a magnetometer chip to calculate position using the Earth's magnetic field.
Look at the relevant work in that last sentence magnetic and now let us see what happens when you put a head torch next to it
By all means Dear Reader use your IPhone to navigate out there but remember they are as susceptible to magnets and electronic interference as their older counterpart the needle compass.