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Steering a Boat by Compass
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Typical Deviation Table
from the geographic North Pole. A magnetic compass points to magnetic north instead of true north because of Earth's magnetic field. The amount the compass is offset  from  the  true  pole  is  called  variation. Variation  differs  at  various  points  on  Earth's surface, and at many points it increases or decreases by a known annual rate. Variation for any given locality, together with the amount of increase or decrease, is shown on the compass rose  of  the  chart  for  that  particular  locality.  On small-scale charts of larger areas, however, variation is shown by lines (isogonic lines) running through points with  the  same  amount  of  variation. Along each line or every fifth line (depending upon the type of chart used), variation is printed, and rates of annual  changes  are  shown  between  the  lines. Figure 5-13 shows a compass rose indicating that in 1968 there was a 26°45' easterly variation in that area and that it was increasing 11' annually. The total amount of variation is found by multiplying the number of years since the year printed in the compass rose by the rate of annual change. The result is either added to or subtracted from the variation given, depending upon whether the error is increasing or decreasing. In this instance, total variation in 1992 would be 24 years x 11' annual increase + 26°45' = 31°09'. Variation remains the same for any heading of a ship or boat at any given locality. No matter which direction your  boat  is  heading,  the  magnetic  compass,  if  affected by  variation  alone,  points  in  the  direction  of  the magnetic  pole. Deviation is caused by the magnetic metallic masses in and on a ship. It is built into a ship and the Figure  5-13.–Combination  compass  rose. 5-16

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