Quantcast Cross-Section Leveling

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Figure 14-18.—Using angle prism for sighting 90° from the center-line  stakes. Cross-Section Leveling In  profile  leveling,  you  determine  the  eleva- tions  of  a  series  of  points  lengthwise  along  a highway. In cross-section leveling, you determine the  elevations  of  points  on  a  succession  of  lines running at right angles to the lengthwise line of the  highway.  The  principal  purpose  of  profile leveling is to provide data from which the depth of  fill  or  cut  required  to  bring  the  existing surface  up  to,  or  down  to,  the  grade  elevation required  for  the  highway  can  be  determined. Note  that  profile  leveling  provides  this  data relative to the center line. In figure 14-16, you can see along the top the depth of cut or fill required at  each  station  to  bring  the  existing  surface  to grade—the prescribed grade line for the highway is  indicated  by  the  smoothly  curved  grade  line shown.  At  each  station,  you  can  determine  the cut  or  fill  by  counting  the  squares  between  the profile  and  the  grade  line. The   cross-section   lines   are   established   at regular  stations,  at  any  plus  stations,  and  at intermediate breaks in the ground. Short cross- lines are laid out by eye, but long crosslines are laid out at a 90° angle to the center line with the transit.  For  short  crosslines,  most  surveyors  prefer to use an angle prism for sighting 900 angles from the  center  line.  Figure  14-18  shows  a  surveyor using an angle prism for sighting a 900 angle from the  center  line  of  the  highway. For  cross-section  leveling,  strip  topography, and  some  other  purposes,  it  is  necessary  to  lay off a 900 angle  at  numerous  points  along  a  line. This   900 angle  can  often  be  established  by 4-18 Figure  14-19.-Laying off a 900 angle from the center-line stakes. estimation   with   sufficient   accuracy   for   the particular  job.  The  surveyor  straddles  the  point on  the  line,  arms  extended  sideward  along  the marked line (fig. 14- 19). By looking alternately right then left, he adjusts the position of his feet until his body is in line with AB. He then brings his hands together in front of him, thus pointing along  an  approximate  900 line  from  the  marked line.  An  experienced  surveyor  can  lay  off  a  900 angle by this method so that a point 100 ft away will  be  within  1  ft  of  the  true  perpendicular. You should measure all elevations at abrupt changes or breaks in the ground with a rod and level. Measure all distances from the center line with a metallic tape. In rough country, the hand level  can  be  used  to  advantage  for  obtaining  cross sections  if  the  center-line  elevations  have  been determined  by  use  of  the  engineer’s  level. Cross-section leveling is usually done with a hand level after the profile run has been made. The  method  is  as  follows: From  the  profile  run,  you  know  the  center- line elevation at each station. Suppose you want to take cross-section elevations at 10 ft intervals for 40 ft on either side of the center line. The first thing you do is to determine the vertical distance from the ground to your line of sight through the hand level when you stand erect with the level at your eye. The best way to do this is to sight on a level rod held plumb in front of you. Suppose you find that the vertical distance is 5.5 ft. Then your HI at any center-line station is the center- line elevation (obtained in the profile level run) plus  5.5  ft.



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