Q. Do magnetic phone holders affect GPS?

Answer

Because it is not something that relies on geomagnetic forces but rather a system that is dependent on satellites it cannot be influenced by a magnetic field or a magnetic phone mount. This is an important feature. Nevertheless weak GPS signals can be affected by big obstructions such as mountains and buildings which might lead them to make errors. But I've observed that GPS fixes are taking a lot longer than they used to and GPS navigation apps are having a hard time staying on the predefined routes and wandering off course a lot more frequently than they used to. This is really frustrating as the conventional method for calibrating a GPS (waving a "8" figure in the air etc.)