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.