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Magnetic Compass
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that  heading.  When  converting  true  heading  to magnetic, subtract easterly errors and add westerly errors. CIRCULAR MEASUREMENT Before we go any further, you must know how distances are measured along the circumference of a circle. Measurement along a meridian, a perfect circle, is expressed in degrees of arc. These degrees of arc may be transformed into linear measurement. The compass card is the best example of circular measurement in degrees of arc. Whatever the size of the card, its circumference always  contains  360°.  Each  degree  contains  60  minutes ('), and each minute contains 60 seconds (''). MAGNETIC COMPASS ERROR LEARNING OBJECTIVE: Explain magnetic compass  error,  including  variations  and deviations. Most of the time the magnetic compass does not point directly north. Usually, there is a difference of several degrees. This difference, known as compass error, is made up of variation and deviation. VARIATION 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. The compass rose shown in figure 2-4 indicates that in 1990 there was a 14°45' westerly variation in that area, increasing  1'  annually. To find the amount of variation in this locality in 1995, 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 of 14°50'  W. 2-3 Figure 2-4— Compass rose. Remember: If the annual variation is an increase, you add; if it is a decrease, you subtract. 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. 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. Some ships may have another magnetic compass, also  known  as  the  emergency  steering  compass,  located at the after steering station, when that station is topside.

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