Navigation and Seamanship

2.1.4. Compass error - I-Variation

Variation

Since the magnetic poles of the earth do not coincide with the geographic poles, a compass needle (in line with the earth’s magnetic field) will not indicate true north. The direction in which it points at any position on the Earth’s surface is known as the direction of the magnetic meridian. The angular difference between the true meridian and the magnetic meridian is called Variation or Magnetic Variation or Declination. Or in other words, Compass needle point to MAGNETIC NORTH, which varies from True North by an error is termed VARIATION. The value of variation should always be suffixed with East or West. This variation has different values at different locations on the earth. The variation values are place and time specific. The variation values could be obtained from compass roses of navigational charts. The name and the value of the Variation at any point on the Earth may be found from the appropriate Magnetic Variation chart. On such a chart a line joining places of equal variation is known as an ISOGONIC LINE. Also the value of variation for that area and the amount it is likely to change in a year is printed in the centre of compass rose of the chart .In order to calculate the variation at a later date, from the annual change in variation printed on the compass rose is useful.

(Since most navigation on a chart involves the use of the compass, a reproduction of a compass card is printed at strategic points across the face of every chart. These are termed compass roses and their positioning is such that there is always one close to hand no matter where on the chart the navigator is working. They are graduated in three-figure notations from 0° to 360° and contain details of the variations in force in that area.)

Any reading or information on a chart including bearings is related to True North only.