One of the most common questions I receive these days is why does the compass not match my Global Positioning System (GPS) readings. There are two reasons for this discrepancy: the earth's magnetic variation and differences in what each instrument measures. While the first problem is well known (and printed on every navigation chart) the second is not obvious to most recreational boaters. A magnetic compass measures the direction the boat is pointed, i.e. its heading. A GPS, on the other hand, takes sequential measurements of the boats position to determine the direction of travel, i.e. its course. A vessel's course is affected by other factors besides the boats heading and speed, including the effect of wind and current. A GPS cannot measure a vessel's heading, unless it also has a compass or gyroscope within the instrument.
An Auto Pilot usually uses a Fluxgate sensor which is sensitive to every movement of the boat, both vertical and horizontal. Using electronic averaging, this sensor attempts to remove error but often does not exactly agree with the magnetic compass, especially when the boat is at rest. (Remember that when you boat is in its house, the distance between the various compasses and sensors vary with the metal of the house and roof, which also can cause the various compasses to read different headings “at home”.)
Poor Auto Pilot Sensor installation is also a common problem that I find when a compass and Auto Pilot disagree. Improper alignment of the sensor components with the vessel's keel is often the cause and is easily correctable. If you need help fixing these problems, please contact me.