Why not use your GPS instead of the hand-bearing compass? Because a GPS isn't magnetic. It works by triangulation, using speed and present position to deliver a direction. It gives you a COG (course over ground), telling you which way you are going, not which way you are pointing. At any given instant the heading may be right, but since it's constantly averaging, you can't be sure. All's usually well at speeds above 10 knots, but when you slow down, as you would in fog, snow, dark, whatever, the differences between small changes at the wheel and what the display shows can be great. This is one reason why a heading sensor — a gyro or an electronic compass — is recommended when installing an autopilot. But all you have to do to see this inconstancy is make for a waypoint and then compare your track to the velocity made good display. You'll see that a GPS actually provides headings in tight wiggles rather than straight lines.