What is a Magnetic Compass, its principle, types and errors?

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.

2. DEVIATION

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  increases,  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.