I have always been fascinated with compasses. It all started with a mineral survey in Canada’s arctic as a summer student. I used a Silva Ranger compass which I still have many years after. Later on I worked in Indonesia and discovered that my compass didn’t work properly in the Southern Hemisphere because it was balanced for the vertical magnetic component in Canada!
Recently I purchased an iPhone10, so I guess it was no accident that one of the first things I did was check out the compass. The application is located in the Utilities icon. Under Settings, you can have it point to Magnetic North or True North. Presumably your Lat/Long will determine an automatic Magnetic Variation adjustment. I tested the compass indoors and outdoors as well in comparison to the Silva Ranger. I noticed that the iPhone compass did not react to a nearby magnet as the Silva did. So I need to do more research to determine what sensor/method is used to implement the compass.
A bearing check on the CN Tower from my home location gives the following results:
Silva Ranger Bearing = 172deg
iPhone Bearing = 171deg
True Bearing = 171.4deg