If the instrument is not level, the measurement of  both  horizontal  and  vertical  angles  will  be inaccurate. 3. The line of sight through the telescope must be perpendicular to the horizontal axis. If it is not, the line of sight through the telescope inverted will not be 1800 opposite the line of sight through the telescope erect. 4.  The  horizontal  axis  of  the  telescope  must be perpendicular to the vertical axis. If it is not, the measurement of both horizontal and vertical angles will be inaccurate. 5.  The  axis  of  the  telescope  level  must  be parallel to the line of sight through the telescope. If  it  is  not,  the  telescope  cannot  be  accurately leveled.  If  the  telescope  cannot  be  accurately leveled,  vertical  angles  cannot  be  accurately measured. 6. The point of intersection of the vertical and horizontal  cross  hairs  must  coincide  with  the  true optical   axis   of   the   telescope.   If   it   doesn’t, measurement   of   both   horizontal   and   vertical angles will be inaccurate. NOTE:  Any  or  all  of  the  above  conditions may be absent in an instrument that is defective or  damaged,  or  one  that  needs  adjustment  or calibration. Identifying Natural Errors Common  causes  of  natural  errors  in  transit work  are  as  follows: 1. Settlement of the tripod in yielding soil. If the tripod settled evenly—that is, if the tip of each leg  settled  precisely  the  same  amount—there would be little error in the results of measuring horizontal angles. Settlement is usually uneven, however,  which  results  in  the  instrument  not being level. 2. Refraction—but the effect of this is usually negligible  in  ordinary  precision  surveying. 3.  Unequal  expansion  or  contraction  of instrument parts caused by excessively high or low temperature.  For  ordinary  precision  surveying,  the effect  of  this  is  also  usually  negligible. 4.   High   wind   may   cause   plumbing   errors when  you  are  plumbing  with  a  plumb  bob  and cord and may also cause reading errors because of  vibration  of  the  instrument. Identifying Personal Errors Personal  errors  are  the  combined  results  of carelessness and of the limitations of the human Figure 13-28.-Exaggerated illustration of error caused when the transit is not centered exactly over the occupied station. eye  in  setting  up  and  leveling  the  instrument  and in  making  observations. Common causes of personal errors in transit work  are  as  follows: 1. Failure to plumb the vertical axis exactly over the station. Figure 13-28 shows how the result of  inaccuracy  increases  drastically  as  the  sight distance decreases. In that figure, an instrument supposed  to  be  set  up  at  A  was  actually  set  up at  A´,  40  ft  away  from  A.  (For  demonstration purposes the figure was exaggerated to magnify the  error;  in  actual  practice  the  eccentricity amounts only to a fraction of an inch. Remember that mathematically, 1 in. is the arc of 1 min when the  radius  is  300  ft.) In  the  upper  view,  you  can  see  that  with  B located  300  ft  from  A,  the  angular  error  caused by the displacement is about 8°. In the lower view, however, with B located only 100 ft from A, the angular  error  caused  by  the  displacement  is  about 22°. The  practical  lesson  to  be  learned  from  this is that you must plumb the instrument much more carefully  for  a  short  sight  than  for  a  long  one. 2. Failure to center plate level bubbles exactly. The  result  of  this  is  that  the  instrument  is  not leveled  exactly.  The  consequent  error  is  at  a minimum for a horizontal sight and increases as a  sight  becomes  inclined. The practical lesson is that you should level the  instrument  much  more  carefully  for  an  incline sight  than  for  a  horizontal  one. 13-25