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RUNNING  A  DISTANCE  (LINE)
RANDOM  LINE

Engineering Aid 3 - Beginning Structural engineering guide book
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is set up at B, and a 90° angle is turned from line AB. The distance BB´ is carefully measured and recorded.  The  instrument  is  moved  to  B´,  and another  90°  angle  is  turned.  B´C´  is  laid  off  to clear the obstacle. The instrument is moved to C, and  a  third  90°  angle  is  turned.  Distance  CC´, equal  to  BB´,   is  measured  and  marked.  This establishes  a  point  C  on  the  original  line.  The instrument is moved to C, and a fourth 90° angle is turned to establish the alignment CD that is the extension  of  AB  beyond  the  obstacle. When the distance to clear the obstacle, BB´ or CC´, is less than a tape length, you can avoid turning four 90° angles as follows: Erect perpen- dicular offsets from points A and B in figure 13-18 so that AA´ equals BB´. Set up the instrument at B’, and measure angle A´B´B to be sure that it’s 90°.  Extend  line  A´B´  to  C´  and  then  to  D´, making  sure  that  point  C  clears  the  obstacle. Then,  lay  off  perpendicular  offset  C´C  equal  to AA´  or  BB´  and  perpendicular  offset  D´D  equal to C´C. Then, line CD is the extension of line AB. The  total  distance  of  the  line  AD  is  the  sum  of the  distances  AB,  B´C´,  and  CD. You  also  compute  the  diagonals  formed  by  the end  rectangles  and  compare  the  result  to  the actual  measurement,  if  you  can,  as  a  further check. Line Between Nonintervisible Points Sometimes  you  need  to  run  a  straight  line between  nonintervisible  points  when  events  make the  use  of  the  above  methods  of  bypassing  an obstacle impractical. If there is an intermediate point on the straight line from which both of the end  points  can  be  observed,  the  method  called BALANCING  IN  (also  called  BUCKING  IN, JIGGLING  IN,  WIGGLING  IN,  or  RANGING IN)  may  be  used. A problem often encountered in surveying is to  find  a  point  exactly  on  the  line  between  two other points when neither can be occupied or when an obstruction, such as a hill lies between the two points. The point to be occupied must be located so that both of the other points are visible from it.  The  process  of  establishing  the  intermediate point  is  known  as  wiggling  in  or  ranging  in. The approximate position of the line between the two points at the instrument station is first estimated  by  using  two  range  poles.  The  range poles  are  lined  in  alternately  in  the  following manner. In figure 13-19, view A, set range pole 1 and move range pole 2 until it is exactly on line between  pole  1  and  point  A.  You  do  this  by Figure 13-19.-Setting up on a line between two points. sighting  along  the  edge  of  pole  1  at  the  station A until pole 2 seems to be on line. Set range pole 2 and move pole 1 until it is on line between pole 2 and point C. Now, move pole 2 into line again, then pole 1, alternately, until both are on line AC. The line will appear to pass through both poles and  both  stations  from  either  viewing  position. After finding the approximate position of the line  between  the  two  points,  you  set  up  the instrument on this line. The instrument probably will not be exactly on line, but will be over a point, such as B´, (see fig. 13-19, view B). With the in- strument at  B´, you  backsight  on  A  and  plunge the telescope and notice where the line of sight C passes the point C. Estimate this distance CC´ and also the distance that  B´ would  be  away  from C  and  A.  Estimate  the  amount  to  move  the instrument to place it on the line you need. Thus, if B´ is midway between  A and C, and C´ misses C by 3 feet to the left,  B´ must  be  moved  about 1.5  feet  to  the  right  to  reach  B. Continue  the sequence of backlighting, plunging the telescope, and moving the instrument until the line of sight 13-16







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