Not so long ago a compass was the most important instrument aboard. The compass was used to find the boat’s position, by taking bearings on two or more objects, and for setting and holding a direction of travel, or course. Today GPS subsumes both of those functions with greater accuracy and convenience, making the compass at most a backup instrument. It is difficult to imagine a situation where you would use the compass while a smart GPS unit is at hand.
Bearings
A bearing is the direction, toward an object, measured in degrees from north. A true bearing is the bearing measured from true north, the direction toward the true North Pole. A magnetic bearing is measured from magnetic north. A compass bearing is the bearing that shows on the compass. These three are generally different.Deviation
A compass is subject to two kinds of error. Deviation, also called declination, is the difference between magnetic north and true north, an angle that varies from place to place, all around the earth, primarily because the magnetic North Pole is not at the same location as the true North Pole. The line of zero deviation (the agonic line) in the US runs from the north magnetic pole through Lake Superior and across the western panhandle of Florida. Along this line, true north is the same as magnetic north. West of the agonic line, a compass will give a reading that is east of true north. East of it, a compass reading will be west of true north.Variation
Variation is pure compass error, the difference between magnetic north and the bearing that the compass shows as north, an angle that usually varies as the bearing changes, primarily because of metal in or near the compass. Thus, deviation depends on location (and is the same in every direction and for every compass), while variation, a property of the compass and the boat, depends on direction (and is the same at every location on the sea, although local variations in the earth’s crust can contribute on land).Deviation can be read from a chart. Variation can be determined by comparing the compass bearing with a GPS bearing that has been converted from true to magnetic. Because both tend to be small, deviation and variation are expressed as some number of degrees east (positive, or clockwise) or west (negative, or counterclockwise) from north. Deviation is the number of degrees that magnetic north lies east or west of true north, and variation is the number of degrees that compass north lies east or west of magnetic north.