Navigation and Seamanship (1+1)

Errors of the magnetic compass

1. Variation:

                The Earth’s rotation axis is not aligned with earth’s magnetic field. The magnetic compass needle will show only the magnetic north of earth. The difference between true north and magnetic north is called ‘Variation’. It is defined as the angle between the true and magnetic meridians. It varies from time to time and from place to place. This is because of the soil property of earth’s crust. The variation is also called earth’s declination. Variation is important to find the true direction of the vessel.

Find out the Variation

Variation of particular place can be referred from nautical chart or magnetic variation chart. In the nautical charts the variation values are printed inside the compass rose. As the variation values are varying with time the annual increase or decrease is marked in the compass rose. The variation values are to be adjusted with annual increase or decrease.

Deviation:

Apart from the variation, the ferrous materials and electronic instruments in the vessel may have influence over the magnetic compass needle.So the magnetic needle further deflected from magnetic north .This cause another error called Deviation. The deflection of the compass needle from magnetic north is knowns as deviation. It may be due to materials such as steel, iron or electrical equipment.       

Compass Error

Compass error is the combined effect of  deviation and  variation. The values of variation and deviation should be suffixed with either Easterly(E) or Westerly(W). If the variation and deviation are of the same Side (both easterly or both westerly), they are added to give the compass error. If they are of different suffixes they are subtracted with the name of direction of the larger of the two being given to the resultant.