Swinging the Compass:

Deviation is the problem of compass error due to structure of the vessel, proximity of metal objects, magnetic fields or electrical equipment. It may be influenced by loading cargo (e.g. canned fruit) or having repair work done (new metal parts or electronics). Deviation also varies with the ship's head (way it is pointing) as this changes the position of the magnetic fields in the boat relative to the earth's magnetic field as well as it's relationship with interfering objects onboard.

Individual vessels have individual deviation cards that show the adjustment required for changes in position of the vessel's head. The process used to produce a deviation card is called 'swinging the compass'. There are several methods for doing this but in general terms the procedure is as follows:

1. The vessel is anchored securely in midstream

2. A transit line of known bearing (magnetic bearing from the chart) is established

3. The vessel moves through each of eight compass bearings (cardinal and inter-cardinal points)

4. For each of the eight points the bearing of the transit line is taken and any discrepancy between this compass bearing and the known chart bearing is noted. If the compass bearing is greater the deviation is west, if less the deviation is east. 

Below is a sample table of deviation values: 

Ship's Head by Compass	Compass Bearing	Magnetic Bearing	Deviation
N	280°	278°	2°W
NE	282°	278°	4°W
E	281°	278°	3°W
SE	279°	278°	1°W
S	276°	278°	2°E
SW	274°	278°	4°E
W	275°	278°	3°E
NW	278°	278°	NIL

This can be plotted as a deviation curve and intermediary values read off the graph if required. 

Total compass error is the combination of both deviation and variation (declination) and the rule is:

LIKE NAMES ADD

UNLIKE NAMES SUBTRACT

For example, a variation of 15° east and a deviation of 5° west, gives a total compass error of 10° east. Whereas a variation of 11° west and deviation of 3° west, gives a total compass error of 14° west.

How Marine Compasses Work Types of Compasses Deviation, Declination and Variation Swinging the Compass