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Holding on a Point
Identifying Points

Engineering Aid 3 - Beginning Structural engineering guide book
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above),  you  need  to  learn  to  brace  your  holding arm with your other arm, and against your body or  head  or  both,  to  avoid  unsteadiness  and fatigue.  When  there  is  a  wind,  you  may  find  it difficult  to  hold  the  plumb  bob  suspended  over a point. The plumb bob will tend to swing back and  forth.  You  can  overcome  this  problem  by bouncing the point of the plumb bob slightly up and  down  on  the  point. For  a  long  sight,  it  is  much  better  to  plumb over a point with a range pole. For a short sight, however, the shaft of a range pole is too thick to permit exact alignment of the vertical cross hair. For long sights, or for sights on a point that is  to  be  sighted  repeatedly,  it  is  often  desirable to  construct  a  semipermanent  target.  There are  no  definite  rules  that  can  be  stated  for constructing  targets  because  they  usually  must  be built from materials at hand. Use your ingenuity; but make the target high enough to be seen, strong enough  to  withstand  prevailing  winds,  and  plumb over  a  point.  Several  types  of  semipermanent targets are shown in figure 12-6. MARKING  CONTROL  POINTS, REFERENCE  POINTS,  AND MONUMENTS In  general,  control  surveys  deal  with  estab- lished  points.  To  define  these  points,  surveyors Figure 12-6.-Field-constructed semipermanent targets. have  to  mark  them.  Certain  points  are  made permanent;    on  the  other  hand,  others  are temporary.  A  line  that  will  be  used  for  a  long period  of  time,  for  example,  may  be  marked  at each  end  with  a  bronze  disk  set  in  concrete,  or with a center-punched metal rod driven flush with the  ground.  For  less  permanent  control  points, wooden  stakes  or  hubs  with  nails,  shiners,  and flaggings  can  be  used. Placing Driven Markers A  DRIVEN  MARKER  must  be  set  exactly vertically on the point it is supposed to mark. If it is driven on a slant, the top of the marker will not  define  the  correct  location  of  the  point.  To drive the marker vertically, first align it vertically; then,  using  a  sledgehammer  or  other  type  of driving implement, strike each blow squarely on the  flat  end  of  the  hub  or  stake. A  wooden  hub  is  normally  driven  to  mark the  exact  horizontal  location  of  a  point,  usually for  the  purpose  of  plumbing  an  instrument over  the  point.  Consequently,  it  is  not  normally necessary   for   the   top   of   a   hub   (or   other markers  used  for  the  same  purpose)  to  extend much above the ground line. The precise location of  the  point  is  marked  by  a  hub  tack,  punch mark,   or   other   precise   marker   driven   or   set in  the  top  of  the  hub.  For  work  on  asphalt roads  or  runways,  you’ll  find  it  easier  to  use flagging or a soda pop top and a nail as a marker; in concrete and other hard surfaces, you can use orange paint or a star-drilled hole plugged with lead.  The  choice  of  markers  to  be  used  depends on  the  surveyor’s  judgment  as  well  as  the  purpose of  the  survey. In frozen or otherwise extra hard ground, use a  bull-point  to  start  a  hole  for  a  stake  or  hub. Remember that the stake or hub will follow the line  of  the  opening  made  by  the  bull-point. Therefore,  if  the  bull-point  is  not  driven  vertically, the  stake  or  hub  will  not  be  vertical  either. Placing Monuments In surveying, a MONUMENT is a permanent object or structure used where a point or station must  be  retained  indefinitely  for  future  reference. It  may  simply  consist  of  a  conspicuous  point carved on an outcrop of a ledge rock or otherwise 12-7







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