What is compass adjustment?
A magnetic compass should align itself steadily to the earth’s magnetic field, pointing to magnetic north. However, when the compass is installed on a boat, metals and electrical equipment may cause magnetic fields that distort the earth’s field at the compass location. The effects of these onboard magnetic fields change with the vessel’s heading. The difference between compass north and magnetic north on the different headings is called the deviation. Compass adjustment is the compensation of these deviating forces with magnetic correctors. For various reasons such as design, location and practical expedience, all the deviation may not be removed. The residual deviation is recorded as a table or a curve of deviation against the compass headings.