Correcting  for  Local  Magnetic  Attraction

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observer is determining the magnetic bearing of the  dotted  line  labeled  Line  of  Sight.  First,  the observer mounts the compass on a steady support, levels   it,   and   waits   for   the   needle   to   stop oscillating.  Then,  the  observer  carefully  rotates the compass until the north-south line on the card lies exactly along the line whose bearing is being taken. The  bearing  is  now  indicated  by  the  needle- point.  The  needlepoint  indicates  a  numerical  value of  40°.  The  card  indicates  the  northeast  quadrant. The  magnetic  bearing  is,  therefore,  N40°E. Correcting  for  Local  Magnetic  Attraction Figure  13-4  shows  the  compass  needle  lying along the magnetic meridian. This means either that  the  compass  is  in  an  area  free  of  “local magnetic  attraction”  or  that  the  effect  of  local attraction has been eliminated by adjusting the compass card as described later. “Local magnetic attraction”  means  the  deflection  of  the  compass needle  by  a  local  magnetic  force,  such  as  that created  by  nearby  electrical  equipment  or  by  a mass  of  metal,  such  as  a  bulldozer.  When  local attraction exists and is not compensated for, the bearing  you  get  is  a  COMPASS  bearing.  A compass  bearing  does  not  become  a  magnetic bearing   until   it   has   been   corrected   for   local attraction.   Suppose,   for   example,   you   read   a compass bearing of N37°E. Suppose the effect of the   magnetic   attraction   of   a   nearby   pole transformer  is  enough  to  deflect  the  compass needle 4° to the west of the magnetic meridian. In  the  absence  of  this  local  attraction,  the compass   would   read   N33°E,   not   N37°E. Therefore,  the  correct  magnetic  bearing  is  N33°E. To  correct  a  compass  bearing  for  local attraction, you  determine  the  amount  and direction  (east  or  west)  of  the  local  attraction. First, set up the compass where you propose to take the bearing. Then, select a distant object that you may presume to be outside the range of any local attraction. Take the bearing of this object. If   you   read   a   bearing   of   S60°W,   shift   the compass  to  the  immediate  vicinity  of  the  object you sighted on; and take, from there, the bearing of the original setup point. In the absence of any local  attraction  at  the  original  setup  point,  you would   read   the   back   bearing   of   the   original bearing  or  N60°E.  Suppose  instead  you  read N48°E.  The  back  bearing  of  this  is  S48°W. Therefore,   the   bearing   as   indicated   by   the compass  under  local  attraction  is  S60°W;  but as  indicated  by  the  compass  not  under  local attraction, it is S48°W. The amount and direction of  local  attraction  are,  therefore,  12°W. The  question  of  whether  you  add  the  local attraction  to,  or  subtract  it  from,  the  compass bearing to get the magnetic bearing depends on (1)  the  direction  of  the  local  attraction  and  (2)  the quadrant the bearing is in. As  a  rule,  for  a  bearing  in  the  northeast quadrant, you add an easterly attraction to the compass bearing to get the magnetic bearing and subtract a westerly  attraction  from the compass bearing to get the magnetic bearing. Now,  consider  the  compass  shown  in  figure 13-5. This compass indicates a bearing of S40°W. Suppose  the  local  attraction  is  12°W.  The needle, then, is 12°W of where it would be without local attraction. You can see that,  in the southwest quadrant, you would subtract westerly attraction and add easterly attraction. From  a  study  of  the  paragraphs  above,  it becomes   obvious   that   the   procedure   is   the opposite for bearings in the northwest or southeast Figure 13-5.-Compass bearing affected by local magnetic attraction. 13-4

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