Figure 7-4.—Differential-level circuit and notes for differential leveling.six-time angles. Each of these must be reduced to themean angle, as explained in the EA3 TRAMAN. Foranother example: field notes may show a succession ofchained slope distances. Unless the order of precisionof the survey permits slope corrections to be ignored,each of these slope distances must be reduced to thecorresponding horizontal distance.In a closed traverse you must attain a ratio of linearerror of closure and a ratio of angular error of closurethat are within the maximums specified for, or impliedfrom, the nature of the survey.An error that is within the maximum allowable iseliminated by adjustment. “Adjustment” means theequal distribution of a sum total of allowable error overthe separate values that contribute to the total. Suppose,for example, that for a triangular closed traverse withinterior angles about equal in size, the sum of themeasured interior angles comes to 179°57´. The angularerror of closure is 03´. Because there are three interiorangles about equal in size, 01´ would be added to themeasured value of each angle.LEVEL COMPUTATIONSIn making level computations, be sure to check thenotes for a level run by verifying the beginning benchmark (BM); that is, by determining that the correct BMwas used and its correct elevation was duly recorded.Then check the arithmetical accuracy with whichyou added backlights and subtracted foresights. Thedifference between the sum of the foresights taken onBMs or turning points (TPs) and the sum of thebacklights taken on BMs or TPs should equal thedifference in elevation between the initial BM or TP andthe final BM or TP. This fact is shown in figure 7-4.You must remember that this checks the arithmeticonly. It does not indicate anything about how accuratelyyou made the vertical distance measurements.Adjusting Intermediate BenchMark ElevationsLevel lines that begin and end on points that havefixed elevations, such as benchmarks, are often calledlevel circuits. When leveling is accomplishedbetween two previously established bench marks orover a loop that closes back on the starting point, theelevation determined for the final bench mark isseldom equal to its previously established elevation.The difference between these two elevations for thesame bench mark is known as the error of closure.The REMARKS column of figure 7-4 indicates thatthe actual elevation of BM 19 is known to be7-5

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