should  check  and  clear  parallax  before  the  first sighting   and   should   not   readjust   it   until   all sightings  from  the  setup  are  complete. 4.   Rod   improperly   plumbed.   This   error   is caused by a rodman who does not pay attention to his work. The instrumentman can call attention to plumbing if it is at a right angle to his line of sight, but he cannot see it in the direction of line of sight. The use of a rod level or waving the rod will  avoid  this  error. 5. Unstable object used for a TP. The rodman causes  this  error  by  selecting  a  poor  point  of support,  such  as  loose  rocks  or  soft  ground.  As the rod is turned between sights, the weight of the rod can shift a loose rock or sink into soft ground. The elevation of the TP as used for the next BS can change appreciably from the value that had been computed from the previous FS. This error can be avoided by using the turning pin or pedestal when  the  ground  does  not  present  solid  points. 6. Rod length erroneous. This error results in either too long or too short rod readings at each point. In a survey predominately over slopes, this error will accumulate. The rod length should be checked with a steel tape at intervals to locate this error. 7.   Unbalanced   BS   and   FS   distances.   The unbalanced  distances  do  not  cause  the  error.  It is caused by the effect on the line of sight from residual adjustment and leveling errors and the effect  of  curvature  and  refraction  errors.  Readings you  take  at  a  long  distance  will  have  a  greater error   than   those   at   a   short   distance.   This unbalance  may  not  be  critical  on  one  setup  but can  be  compounded  into  a  considerable  error  if the unbalance continues over several setups. By balancing the sight distances at each instrument setup, if possible, and the sums of the BS and FS distances at every opportunity, you will keep these errors  to  a  minimum. 8. Earth’s curvature. This produces an error only on unbalanced sights in leveling. When the BS   distances   are   constantly   greater   than   FS distances,  or  vice  versa,  a  greater  systematic error results, especially when the sights are long. To  eliminate  this  error,  you  must  maintain  a balanced   sight   distance   in   every   BS   and   FS reading, not just their sum total between BMs (the error varies directly as the square of the distance from  the  instrument  to  the  rod). 9.   Atmospheric   refraction.   This   error   also varies as the square of the distance but opposite in  sign  (  +  or  –  )  to  that  caused  by  the  earth’s curvature.  The  effect  of  atmospheric  refraction is only one-seventh of that caused by the earth’s curvature. In first- and second-order leveling, the effect of refraction is minimized by taking the BS and FS readings in quick succession and avoiding readings near the ground. (They should be taken at  least  2  ft  from  the  ground.) 10. Variation in temperature. If a portion of the telescope is shaded and some parts are exposed to the sun’s rays, it produces some warping effect on the instrument that may affect its line of sight. This effect is negligible in ordinary leveling; but in  leveling  of  higher  precision,  this  effect  may produce  appreciable  error.  This  is  one  of  the reasons why surveyors use an umbrella to shield the  instrument  when  doing  more  refined  work. BASIC  ENGINEERING  SURVEYS AND CONSTRUCTION SITE SAFETY An  engineering  survey  forms  the  first  of  a chain  of  activities  that  will  ultimately  lead  to  a completed   structure   of   some   kind,   such   as   a building, a bridge, or a highway. An engineering survey  is  usually  subdivided  into  a  DESIGN- DATA   SURVEY   and   a   CONSTRUCTION SURVEY. This  section  discusses  the  basic  engineering surveys  commonly  performed  by  an  EA  survey party in support of military construction activities. In  addition,  various  types  of  occupational  hazards relating  to  specific  surveying  operation  are  also presented  in  this  section  together  with  the precautions  or  applicable  abatement  procedures that  must  be  carried  out  to  deter  injury  to  the survey  crew  and/or  damage  to  surveying  equip- ment  or  material. HIGHWAY   SURVEYS Surveys for roads and streets involve both field work  and  office  work.  The  extent  of  each  type of  work  depends  on  the  magnitude  and  complexity of the job. Some phases of the work may be done either  in  the  field  or  in  the  office,  and  the decision as to the exact procedures to be followed will  be  influenced  by  the  number  of  personnel available and by the experience and capabilities of  the  individuals  involved. Design-Data Survey This  type  of  survey  is  conducted  for  the purpose of obtaining information that is essential for  planning  an  engineering  project  or  develop- ment and estimating its cost. A typical design-data 14-25