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Figure  13-10.-Setting  the  vernier  at  zero-zero.
Measuring Vertical Angles

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure  13-12.-Sample  field  notes  from  a  deflection  angle  transit-tape  survey. 2. Read the vernier with the eye directly over the top of the coinciding graduations to eliminate the  effects  of  parallax. 3. Take the reading of the other vernier as a check.  The  readings  should  be  180°  apart. 4. Check the plate bubbles before measuring an angle to see if they are centered, but do not disturb the leveling screws between the initial and final  settings  of  the  line  of  sight.  If  an  angle  is measured again, the plate may be releveled after each  reading  before  sighting  again  on  the  starting point. 5. Make sure that the rodman is holding the range  pole  truly  vertical  when  you  sight  at  it. When the bottom of the range pole is not visible, let  the  rodman  use  a  plumb  bob. 6. Avoid accidental movement of the horizon- tal circle; for instance, moving the wrong clamp or  tangent  screw.  If  a  number  of  angles  will  be observed  from  one  setup  without  moving  the horizontal circle, you should sight at some clearly defined distant object that will serve as a reference mark  and  take  note  of  the  angle.  Occasionally, you  should  recheck  the  reading  to  this  point during  measurement  to  see  if  there  is  any accidental  movement. An  example  of  a  horizontal  deflection  angle measurement is shown in figure 13-12. The field notes  contain  data  taken  from  a  loop  traverse shown in the sketch. The transit was first set up at station A, and the magnetic bearing of AB was read  on  the  compass.  Then  the  deflection  angle between the extension of EA and AB was turned in  the  following  manner: 1. The instrumentman released both clamps, matched  the  vernier  to  zero  by  hand,  tightened the upper motion clamp, and set the zero exactly with the upper tangent screw. 2.   With   the   telescope   plunged   (inverted position),  the  instrumentman  sighted  the  range pole  held  on  station  E.  Then  he  tightened  the lower  motion  clamp  and  manipulated  the  lower motion tangent screw to bring the vertical cross hair  to  exact  alignment  with  the  range  pole. 3.  The  instrumentman  replunged  the  telescope and trained on the extension of EA. (Notice that the  telescope  is  in  its  normal  position  now.)  He then released the upper motion and rotated the telescope to the right until the vertical cross hair came  into  line  with  the  range  pole  held  on station B. He further set the upper motion clamp screw  and  brought  the  vertical  cross  hair  into exact alignment with the range pole by manipu- lating  the  upper  motion  tangent  screw. 4.  The  instrumentman  then  read  the  size  of the  deflection  angle  on  the  A  vernier  (89°01´). Since  the  angle  was  turned  to  the  right,  he recorded  89°01´R  in  the  column  headed  “Defl. Angle.” Likewise,   he   recorded   the   chained distance  between  stations  A  and  B  and  the magnetic bearing of traverse line AB under their appropriate  headings. 13-11







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