Compass_errors.doc

COMPASS ERRORS

The Compass Error of the ship is comprised of two parts:

·         Variation,

·         Deviation.

VARIATION   Caused by the Earth’s magnetic field. Variation is the angle between True North and Magnetic North. Changes value and name depending on ship’s position on the Earth. Variation is obtained from Variation Charts or from the Compass Rose on the Navigational Chart in use. Variation is subject to an annual rate of change and can also be affected by ‘Local Magnetic Anomalies’, both of which are indicated on the Chart.

DEVIATION   Caused by the ship’s magnetic field. Deviation is the angle between Magnetic North and Ship’s Compass North. Changes value and name depending on ship’s heading. Deviation is obtained from the ship’s Deviation Card, entering the table with the ‘Ship’s Head’.

If Variation and / or Deviation is West it is subtracted from the Compass Course / Bearing to obtain the True Course / Bearing.               ‘Error West Compass Best’.

If Variation and / or Deviation is East it is added to the Compass Course / Bearing to obtain the True Course / Bearing.                             ‘Error East Compass Least’.

‘CADET’   Compass To True Add East.

When converting from ‘Compass’ to ‘True’ the Deviation Card must be entered using the Compass Heading

When converting from ‘True’ to ‘Compass’ the Deviation Card must be entered using the Magnetic Heading

Ship Course (°C)

              Deviation                            ( + E,  - W )

              Magnetic     (°M)                                                               

              Variation                            ( + E,  - W )              Ship Course (°T)               

Ship Course (°T)

              Variation                            ( - E,  + W )              Magnetic (°M)

              Deviation                            ( - E,  + W )

              Ship Course (°C)

Compass Errors should be obtained:

·         At least once a watch.

·         After every alteration of course. (Alteration of course affects the Deviation)

·         Whenever any structure / cargo moved on the ship. (This can affect ship’s magnetic field)

·         If Variation changing rapidly.

·         If ship passing through ‘Magnetic Anomaly’ area.

·         If in any doubt as to the reliability of the Ship’s Compass.

Methods of obtaining Compass Errors:

·         Celestial Bodies. (Azimuths, Amplitudes or Polaris)

·         Horizontal Angles. (Sextant or Bearings)

·         Transits.

Azimuth Ring Checks:

·         Free play of the Ring.

·         Eccentricity of Ring.

·         Free play of Prism.

·         Misalignment of Prism. (check using ‘arrow up’ and ‘arrow down’)

·         Misalignment of Sighting Mark.