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Correcting  for  Local  Magnetic  Attraction
Orienting a Compass

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure  13-6.-Magnetic  declination  (west). quadrants. In these quadrants, you add westerly attraction and subtract easterly attraction to the compass  bearing  to  get  the  magnetic  bearing. Determining Magnetic Declination and Dip The angle between the true meridian and the magnetic   meridian   is   MAGNETIC   DECLINA- TION. If the north end of the compass needle is pointing  to  the  east  of  the  true  meridian,  the declination is said to be east. If the north end of the compass needle is pointing to the west of the true meridian, the declination is said to be west. (See  fig.  13-6.) The  magnetic  needle  aligns  itself  with  the earth’s   magnetic   field   and   points   toward   the earth’s  magnetic  pole.  In  horizontal  projections, these lines incline downward toward the north in the Northern Hemisphere and downward toward the south in the Southern Hemisphere. Since the bar  takes  the  position  parallel  with  the  lines  of force,   it   inclines   with   the   horizontal.   This phenomenon  is  the  MAGNETIC  DIP. Converting Magnetic Bearings to True Bearings When for   local you have corrected a compass bearing attraction,   you   have   a   MAGNETIC BEARING.   As   explained   previously,   in   most areas  of  the  earth,  a  magnetic  bearing  differs from  a  true  bearing  by  the  amount  of  the  local magnetic  declination  (called  magnetic  variation by navigators). The amount and direction of local declination  are  shown  on  maps  or  charts  of  the area  in  a  format  similar  to  the  following: “Magnetic  Declination  26°45°W  (1968),  Annual Increase 11´. ”  This means, if you are working in 1988 (20 years later), the local declination is To  convert  a  magnetic  bearing  to  a  TRUE BEARING,  you  apply  the  declination  to  the magnetic bearing in precisely the same way that you apply local attraction to a compass bearing. If   the   declination   is   east,   it   is   added   to northeast and southwest magnetic bearings, and it  is  subtracted  from  southeast  and  northwest magnetic  bearings.  If  the  declination  is  west,  it is  added  to  southeast  and  northwest  magnetic bearings  and  subtracted  from  northeast  and southwest  magnetic  bearings. When you have a compass bearing and know both  the  local  attraction  and  the  local  declination, you can go from compass bearing to true bearing in a single process by applying the ALGEBRAIC SUM  of  local  attraction  and  local  declination, Suppose that local attraction is 6°W and declina- tion, 15°E. You could correct for local attraction and  convert  from  magnetic  to  true  in  the  same operation by applying a correction of 9°E to the compass  bearing. Uncorrecting and Unconverting You correct a compass bearing to a magnetic bearing  by  applying  the  local  attraction.  You convert a magnetic bearing to a true bearing by applying  the  local  declination. At  some  time,  you  may  be  given  a  magnetic bearing  and  have  to  figure  the  corresponding compass  bearing  by  using  both  local  attraction and  local  declination. The terms used to describe these calculations are, for the want of any better expressions, UN- CORRECTING and UNCONVERTING. All YOU need   to   remember   is   that,   when   you   are uncorrecting  or  unconverting,  you  apply  local attraction and local declination in the REVERSE of the directions in which you apply them if you were  correcting  or  converting. 13-5







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