TM  5-6675-302-14&P Section  III.  CONTROLS  AND  INSTRUMENTS 2-7.  General   This   section   describes,   locates,   il- 2-8.  Controls  and  Instruments.  Refer  to  figure lustrates,   and   furnishes   sufficient   information 2-11 for the purpose and location of control and in- about the controls used to operate the target set. struments   for   model   242406,   or   figure   2-12   for model  USATS-79. Section   IV.   OPERATION   UNDER   USUAL   CONDITIONS 2-9. General. a. The  instructions  in  this  section  are  published for  the  information  and  guidance  of  the  personnel responsible  for  the  operation  of  the  surveying target  set. b.  The   operator   must   know   how   to   perform every  operation  of  which  the  target  set  is  capable. This  section  contains  instructions  on  centering  the optical   plummet,   centering   below   apex   mark, checking   the   optical   plummet   and   checking   the target.  Since  nearly  every  job  presents  a  different problem, the operator may have to vary given pro- cedures to fit the individual job. 2-10. Target Set Operation. a.   Setting   Up   Tripod.   For   angular   meas- urements,  the  instruments  must  be  set  up  over  a station  of  the  terrain  which  has  been  determined previously. The bayonet socket of the plumb line is plugged  into  the  bridge  screw  from  below  and secured by a quarter turn to the right. The tripod is then  set  up  so  that  the  plumb  bob,  which  hangs from  the  middle  of  the  central  opening,  points  to the  station  point  from  within  1/2  inch,  and  the tripod’s   plate   is   approximately   horizontal.   To achieve  this,  the  leg,  whose  turning  axis  at  the tripod  plate  is  inclined  most,  must  be  displaced laterally.  Any  other  method  will  affect  the  center- ing. The leg points are set firmly into the soil, with care  being  taken  to  maintain  the  centering  within 1/2  to  1  inch.  Now  the  plumb  bob  maybe  removed if    the    final    centering    is    to    be    done    op- tically.  Otherwise,  in  calm  weather  the  plumb  bob may be used on its own. b.  Securing  Base  Plate  of  Tribrach.  The  tri- brach  itself  consists  of  a  star-shaped  base  plate, where  the  three  leveling  screws  (fig.  2-11  or  2-12) are  kept  fast  by  a  rotatable  spring  plate.  The  spr- ing  plate  is  normally  held  in  position  from  below by  means  of  a  lock  screw.  Withdrawing  this  screw allows the spring plate to be rotated a few degrees. The  base  plate  is  thus  released  and  can  be separated  from  the  tribrach.  For  normal  use,  the spring  plate  should  always  be  secured  to  prevent the tribrach from falling out. c. Fixing and Leveling the Tribrach.  The tri- brach  is  fixed  to  the  tripod  with  the  bridge  screw. The  circular  level  bubble  is  centered  by  turning the leveling screws. d. Fixing the Target.  Turn the tribrach locking lever  (fig.  2-11  for  model  242406,  or  fig.  2-12  for model   USATS-79)   towards   the   right.   Insert   the feet  of  the  target  into  the  corresponding  holes  of the  tribrach  and  turn  lever  firmly  towards  the  left until  it  snaps  over  the  clutch  and  holds  the  target fast. 2-11. Centering with Optical Plummet. a. The eyepiece of the optical plummet (fig. 2-11) must be turned until its plumbing mark appears in focus.  At  the  same  time  the  station  mark  on  the ground  will  be  seen,  over  which  the  instrument must  be  centered  for  night  or  dark-day  operation (para  2-4). b. Loosen  the  bridge  screw  of  the  tripod  slightly to allow the tribrach to be shifted until the ground mark appears in the center of the plumbing mark. c. During  the  shifting,  the  bubble  in  the  circular level must be maintained exactly in the center. d. Tighten the bridge screw. e.  Center  the  target  level.  Turn  the  target through  90°  and  center  the  level  bubble  with  the third  leveling  screw.  Repeat  this  procedure  until the bubble remains central throughout a 360° rota- tion  of  the  target.  The  bubble  should  now  be centered in any position. If not, adjust the circular level. 2-12. Centering Below Apex Mark.  In  galleries, the  station  points  are  often  marked  by  hooks cemented in the roof, In this case, fix a plumb line on the hook, so that the point of the plumb will be just  at  the  height  of  the  upper  center  mark  of  the target. Shift the target on the tripod until its mark is  vertically  below  the  plumb  point  when  the  bub- ble  of  the  circular  level  is  centered,  then  level  the target with reference to the target level. 2-11