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Tripod
Optical Plumbing Assembly

Engineering Aid 3 - Beginning Structural engineering guide book
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pivot point. When the two legs look about as far away from the mark as the third one and all three are about equally spaced, you lower the two legs and press them into the ground. Make any slight adjustment to level the head further by moving the third leg a few inches in or out before pressing it  into  the  ground. On  smooth  or  slippery  paved  rock  surfaces, you  should  tighten  the  tripod  legs  hinges  while setting up to prevent the legs from spreading and causing the tripod to fall. You should make use of  holes  or  cracks  in  the  ground  to  brace  the tripod.  In  some  cases,  as  a  safety  factor,  you should tie the three legs together or brace them with rock or bushes after they are set to keep them from  spreading.  If  setups  are  to  be  made  on  a slippery  finished  floor,  rubber  shoes  may  be  fitted to the metal shoes, or an equilateral triangle leg retainer  may  be  used  to  prevent  the  legs  from sliding. When  you  are  setting  up  on  steeply  sloping ground,  place  the  third  leg  uphill  and  at  a  greater distance  from  the  mark.  Set  the  other  two  legs as  before,  but  before  releasing  them,  check  the stability of the setup to see that the weight of the instrument and tripod head will not overbalance and  cause  the  tripod  to  slip  or  fall. Proper care must be observed in handling the tripod. When the legs are set in the ground, care must be taken to apply pressure longitudinally. Pressure  across  the  leg  can  crack  the  wooden pieces.  The  hinge  joint  should  be  adjusted  and  not overtightened to the degree that it would cause strain  on  the  joint  or  strip  or  lock  the  metal threads. The machined tripod head is to be kept covered  with  the  head  cover  or  protective  cap when  not  in  use,  and  the  head  should  not  be scratched  or  burred  by  mishandling.  When  the tripod is in use, the protective cap is to be placed in  the  instrument  box  to  prevent  it  from  being misplaced   or   damaged.   Any   damage   to   the protective  cap  can  be  transferred  to  the  tripod head. Mud, clay, or sand adhering to the tripod has to be removed, and the tripod is to be wiped with  a  damp  cloth  and  dried.  The  metal  parts should be coated with a light film of oil or wiped with  an  oily  cloth.  Foreign  matter  can  get  into hinged  joints  or  on  the  machined  surfaces  and cause wear. Stability is the tripod’s greatest asset. Instability,  wear,  or  damaged  bearing  surfaces  on the tripod can evolve into unexplainable errors in the final survey results. Range Pole A  range  pole  (also  called  a  lining  rod)  is  a wood or metal pole, usually about 8 ft long and about 1/2 to 1 in. in diameter; it is provided with a steel point or shoe and painted in alternate bands of red and white to increase its visibility. Figure 11-39 shows a variety of range poles. The range pole is held vertically on a point or plumbed over a  point,  so  the  point  may  be  observed  through an   optical   instrument.   It   is   primarily   used as  a  sighting  rod  for  either  linear  or  angular measurements.  For  work  of  ordinary  precision, chainmen may keep on line by observing a range pole.   A   range   pole   may   also   be   used   for approximate  stadia  measurement. Plumb Bob, Cord, and Target A  plumb  bob  is  a  pointed,  tapered  brass  or bronze weight that is suspended from a cord for the  general  purpose  of  determining  the  plumb  line from a point on the ground. Common weights for Figure 11-39.-Range poles. 11-39







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