Errors of magnetic compass:-
The true North Pole and the magnetic north pole are not located at the same spot. This variation causes a magnetic compass needle to point more or less away from true north. The amount the needle is offset is called variation because the amount varies at different points on Earth’s surface. Even in the same locality variation usually does not remain constant, but increases or decreases at a certain known rate annually.
The variation for any given locality, together with the amount of annual increase or decrease, is shown on the compass rose of the chart for that particular locality.
Remember: If the annual variation is an increase, you add; if it is a decrease, you subtract
How to calculate variation from compass rose.
To find the amount of variation in this locality in1995, count the number of years since 1990 (in this case 5); Multiply that by the amount of annual increase;(which here gives you 5 X 1′, or 5); add that to the variation in 1990 and you have a 1995 variation of14°50′ W
Variation remains the same for any heading of the ship at a given locality. No matter which way the ship is heading, the magnetic compass, if affected only by variation, points steadily in the general direction of the magnetic north pole.
The amount a magnetic compass needle is deflected by magnetic material in the ship is called deviation.Although deviation remains a constant for any given compass heading, it is not the same on all headings. Deviation gradually inc
reases, decreases, increases, and decreases again as the ship goes through an entire 360° of swing.
The magnetic steering compass is located in the pilothouse, where it is affected considerably by deviation. Usually the standard compass is topside, where the magnetic forces producing deviation are not as strong. Courses and bearings by these compasses must be carefully differentiated by the abbreviations PSC (per standard compass), PSTGC (per steering compass), and PGC (per gyrocompass). The standard compass provides a means for checking the steering compass and the gyrocompass.