I agree that simpler visual methods are preferable. Smartphone GPS accuracy is 3 meters under good conditions (not the higher numbers some have noted) but that's still not nearly good enough. You can add BlueTooth GPS hardware to iPhones to give sub-meter GPS accuracy, however, it is slow and takes a while to settle on a fix with that level of accuracy. When in motion, that level of accuracy would never be attained.
Smartphone compass (magnetometer) hardware is also fairly lousy. You're lucky to get +/- 10 degree accuracy under good conditions, and if you're near any source of electromagnetic interference (like the dashboard of a car or tractor) it can be much worse.