. Railroad construction. Theory and practice . esented in the section being taken, runs out. Forexample, in Fig. 52, the break on the left of section A, at adistance of yi from the center, is observed to run out in sectionA/ at a distance of yi from the center. The volume of theprismoid, computed by the prismoidal formula as in § 70, willinvolve the midsection, to obtain the dimension of which wouldrequire a laborious computation. A simpler process is to com-pute the volume by averaging end areas as in § 81 and apply aprismoidal correction. To do this write out an expression foreach end area s
. Railroad construction. Theory and practice . esented in the section being taken, runs out. Forexample, in Fig. 52, the break on the left of section A, at adistance of yi from the center, is observed to run out in sectionA/ at a distance of yi from the center. The volume of theprismoid, computed by the prismoidal formula as in § 70, willinvolve the midsection, to obtain the dimension of which wouldrequire a laborious computation. A simpler process is to com-pute the volume by averaging end areas as in § 81 and apply aprismoidal correction. To do this write out an expression foreach end area similar to that given in Eq. 61. The sum of these areag times ^ gives the approximate volume. As before, 96 RAILROAD CONSTRUCTION. §83. for partial station lengths, multiply the result by — ^^^ . There will be no constant sub tractive term, ffa/j, as in § true prismoidal correction may be computed, as in § 83, orthe following approximate method may be used: Consider theirregular section to be three-level ground for the purpose of. Fig. 52. computing the correction only. This has the advantage of lesslabor in computation than the use of the true prismoidal correc-tion, and although the error involved may be considerable inindividual sections, the error is as likely to be positiA^e as nega-tive, and in the long run the error will not be large and generallywill be much less than would result by the neglect of any pris-moidal correction. 83. True prismoidal correction for irregular prismoids. Asintimated in § 82, each cross-section should be assumed to havethe same number of sides as the adjacent cross-section whencomputing the prismoidal correction. This being done, it per-mits the division of the whole prismoid into elementary triangu-lar prismoids, the dimensions of the bases of which being givenin each case by a vertical distance above grade line and by thehorizontal distance between two adjacent breaks. The summa-tion of the prismoidal corrections for of th
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