The night my GPS heading way way off I had the unit upright and it was near a radio. I think both conditions could have thrown it off. However, I like to have the GPS upright so I can see it when I am at the wheel and since I charter different boats who knows about differet magnetic forces in various cockpits. As I mentioned previously, the GPS comes from the factory set so that at speeds <10 mph it uses the electronic compass for heading and at speeds >10 mph it uses the GPS fix from satellites. However, one can change this default so that the unit uses the GPS and not the electronic compass all the way down to 0 mph. I just do not understand GPS units well enough to figure out why they even build in the option, if one would lose accuracy using the GPS as the determinant of heading at slow speeds like 1 or 2 mph, etc.