This paper proposes a proper compass adjustment method using only a GPS (or any other GNSS receiver) and a single visual reference to enhance the efficiency of compass adjustment. During compass adjustment, the ship proceeds on magnetic courses using a gyroscopic or satellite compass and considering magnetic declination. However, non-magnetic compasses are only compulsory for ships of 500 gross tonnage or upwards (SOLAS V/19.2.5.1). Many ships of less than 500 gross tonnage have only a magnetic compass to indicate heading. In these cases, a minimum of five leading lines or a minimum of five bearings of conspicuous and distant points or sun azimuths are necessary to adjust the compass. This makes compass adjustment more laborious and time consuming. To expedite this process, a reliable and practical method was developed to use the courses over ground given by a GNSS receiver and a single visual reference instead of the headings provided by a gyroscopic or satellite compass. The method is valid for all ships, but is primarily intended for those equipped with only a magnetic compass to indicate heading.

Many ship accidents have arisen from an error in course indication. Bearing in mind that the actual errors in gyrocompass and satellite compass are really minor, they may be considered valid to be input into an autopilot provided that any failure in such devices is controlled by means of a secondary heading source such as a magnetic compass. However, magnetic compass deviation may be significant and its heading should be corrected before being input to the autopilot. The errors caused by the geographic variability of the deviation should also be taken into account. Moreover, the current way to reduce the deviation requires that the ship is un-berthed to execute a complete swing. The aim of this article is to obtain a ship magnetic model by means of an algorithm based on least squares to correct magnetic compass heading input in the autopilot and to permit definitive magnetic compass compensation without swinging the ship through 360°.