TBM 16. The BS reading of +6.659 is added tothe elevation of BM 35 and gives the resulting HI(139.822). The rod is moved to Peg 16 (which laterbecomes TBM 16). The FS reading of –4.971 issubtracted from the HI to get the elevation of Peg16. Note that the distance (220 ft each way) is alsorecorded for balancing. The process continuesuntil BM 19 is reached.LEVEL COMPUTATIONS.— ln making levelcomputations, you should be sure to check on thenotes for a level run by verifying the beginningBM; that is, by determining that you used thecorrect BM and recorded its correct elevation, asrequired.Then, you should check on the arithmeticalaccuracy with which you added BSs and subtractedFSs. The difference between the sum of the BSstaken on BMs or TPs and the sum of the FSstaken on BMs or TPs should equal the differencein elevation between the initial BM or TP and thefinal BM or TP.Balanced BS and FS distances are shown infigure 14-14. The distance used for the firstinstrument setup was 220 ft. The first BS (rodreading on El 35) was 6,659 ft. The first FS (rodreading on 0 16) was 4.971. Notice that the plussign (+) appears at the top of the BS column andthat the minus sign (–) appears at the top of theFS column in the field notebook. This helps youto remember that BSs are added and FSs aresubtracted as you compute the new elevations.The BS taken on a point added to the elevationof the point gives the HI. This establishes theelevation of the line of sight so that an FS canthen be taken on any point (BM, TBM, or TP).The level line is extended as far as desired withas many instrument setups as may be necessaryby a repetition of the process used in the firstsetup.The elevation of El 35 is 133.163 ft. The firstHI is133.163 + 6,659 = 139.822 ft.The FS subtracted from the HI,139.822 – 4.971 = 134.851 ft,gives the elevation of 0, the first established.Following through with a similar computation foreach setup, notice that the elevation of El 19 wasfound to be 136.457 ft.Look now at the notes in figure 14-14. Thesum of all the BSs is 24.620 ft. The sum of allthe FSs is 21.326 ft. The difference between thesum of the BSs and the sum of the FSs is24.620 – 21.326 = 3.294 ft.This difference should agree with the differencebetween the actual elevation of BM 35 and theelevation already found for BM 19; that is,136.457 – 133.163 = 3.294 ft.This provides a check on the step-by-stepcomputation of elevations.ADJUSTMENT OF INTERMEDIATE BENCHMARK ELEVATIONS.— Level lines that beginand end on points that have fixed elevations, suchas BMs, are often called level circuits. Whenleveling is accomplished between two previouslyestablished BMs or over a loop that closes backon the starting point, the elevation determined forthe final BM will seldom be equal to its previouslyestablished elevation. The difference betweenthese two elevations for the same BM is knownas the ERROR OF CLOSURE. The Remarkscolumn of figure 14-14 indicates that the actualelevation of BM 19 is known to be 136.442 ft. Theelevation found through differential leveling was136.457 ft. The error of closure of the levelcircuit is136.457 – 136.442 = 0.015 ft.It is assumed that errors have occurredprogressively along the line over which the levelingwas done so that adjustments for these errors aredistributed proportionally along the line as shownby the following example: Referring to figure14-14, you will notice that the total distancebetween BM 35 and BM 19, over which the lineof levels was run, was 2,140 ft. The elevation onthe closing BM 19 was found to be 0.015 ft greaterthan its known elevation. You must thereforeadjust the elevations found for the intermediateTBMs 16, 17, and 18.The amount of correction is calculated asfollows:TBM 16 is 440 ft from the starting BM. The totallength distance between the starting and closingBMs is 2,140 ft. The error of closure is 0.015 ft.14-14

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