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LEVEL AND TRAVERSE COMPUTATIONS - 14070_124
Calculating the Allowable Error - 14070_126

Engineering Aid 1 - Advanced Structural engineering guide book
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Figure 7-4.—Differential-level circuit and notes for differential leveling. six-time  angles.  Each  of  these  must  be  reduced  to  the mean angle, as explained in the EA3 TRAMAN. For another example: field notes may show a succession of chained  slope  distances.  Unless  the  order  of  precision of the survey permits slope corrections to be ignored, each of these slope distances must be reduced to the corresponding  horizontal  distance. In a closed traverse you must attain a ratio of linear error of closure and a ratio of angular error of closure that are within the maximums specified for, or implied from, the nature of the survey. An error that is within the maximum allowable is eliminated by adjustment. “Adjustment” means the equal distribution of a sum total of allowable error over the separate values that contribute to the total. Suppose, for example, that for a triangular closed traverse with interior  angles  about  equal  in  size,  the  sum  of  the measured interior angles comes to 179°57´. The angular error of closure is 03´. Because there are three interior angles about equal in size, 01´ would be added to the measured value of each angle. LEVEL  COMPUTATIONS In making level computations, be sure to check the notes  for  a  level  run  by  verifying  the  beginning  bench mark (BM); that is, by determining that the correct BM was used and its correct elevation was duly recorded. Then check the arithmetical accuracy with which you added backlights and subtracted foresights. The difference  between  the  sum  of  the  foresights  taken  on BMs  or  turning  points  (TPs)  and  the  sum  of  the backlights  taken  on  BMs  or  TPs  should  equal  the difference in elevation between the initial BM or TP and the final BM or TP. This fact is shown in figure 7-4. You must remember that this checks the arithmetic only. It does not indicate anything about how accurately you  made  the  vertical  distance  measurements. Adjusting  Intermediate  Bench Mark Elevations Level lines that begin and end on points that have fixed  elevations,  such  as  benchmarks,  are  often  called level   circuits.   When   leveling   is   accomplished between two previously established bench marks or over a loop that closes back on the starting point, the elevation  determined  for  the  final  bench  mark  is seldom equal to its previously established elevation. The difference between these two elevations for the same bench mark is known as the error of closure. The REMARKS column of figure 7-4 indicates that the  actual  elevation  of  BM  19  is  known  to  be 7-5







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