A GPS provides COG (course over ground). COG is calculated from historical position data to give average COG.
The magnetic compass gives you real-time heading for your boat.
COG and Heading are both an integral part of navigation. Conditions such as wind, tide, current and even physical properties of your boat can cause differences between COG and Heading. Heading is the only reliable reference to steer your vessel accurately under all conditions.
Check out our Resource Center for additional information.