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Piloting - 14220_223
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Quartermaster 1 & C - Military manual for the Quartermaster rate
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Determining the Ship’s Position Using True Bearings - 14220_225
Lines of Position and Fixes Defined A  line  of  position  (LOP)  is  a  line  established  by  observations  or measurement  on  which  a  vessel  can  be  expected  to  be  located.  The concept  of  a  LOP  is  extremely  important  in  piloting.  From  a  single LOP,  one  can  safely  assume  that  the  ship  is  located  somewhere  along that line. A  LOP  may  be  straight  (for  bearings)  or  curved  (for  ranges).  To  obtain a  high  degree  of  accuracy  when  fixing  the  ship’s  position,  you  must  use three  or  more  LOPs. Accuracy:  Factors  such  as  chart  errors,  human  limitations,  and equipment  errors  may  affect  the  accuracy  of  a  LOP.  The  accuracy  of any  single  LOP  can  be  checked  by  comparison  with  two  or  more  LOPS that  are  taken  simultaneously.  A  bearing  will  be  suspect  if  it  plots  away from  two  other  LOPs. Fixes A  fix  is  the  point  of  intersection  of  two  or  more  simultaneously  obtained LOPS  indicating  your  ship’s  exact  position  on  the  chart.  The  accuracy of  a  fix  obtained  from  two  LOPS  is  almost  always  questionable. Since we  have  already  stated  that  accuracy  is  of  the  utmost  importance  in piloting,  you  must  always  strive  to  obtain  three  or  more  lines  of  position for  an  accurate  fix. Labeling  LOPs Any  single  LOP  that  is  obtained  must  be  labeled  with  the  time  that  it was  obtained.  This  is  necessary  if  it  is  to  be  in  a  running  fix.  An unlabeled  LOP  can  mistakenly  be  used  and  become  a  source  of  error. In  the  practice  of  piloting,  single  LOPS  are  not  common  because bearings  on  objects  are  taken  at  the  same  time  by  the  bearing  takers. The  fix  resulting  from  these  bearings  is  labeled  with  the  time  the bearings  were  taken. Selecting The  angle  between  selected  objects  is  the  most  important  factor  to Landmarks  for consider  when  selecting  objects  to  fix  the  ship’s  position  from.  For LOPS three  simultaneous  LOPS  to  provide  the  best  fix,  they  should  be  located 120°  apart.  This  is  often  impossible  in  practical  application.  Normally, the  assistant  navigator  or  plotter  will  select  possible  objects  to  obtain bearing  and  the  resulting  fixes  from.  Use  of  conspicuous  landmarks  is always  desirable.  Lighted  towers,  water  tanks,  and  buildings  are  some possibilities. 8-20

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