. Railroad construction. Theory and practice . xr , This condition is fulfilled when the slope isuniform, when a = a. When this is nearly so the erroris evidently not large. On the other hand, if the slopes are in-clined in opposite directions the error may be very considerable,particularly if the angles of slope ate also large. 78. Three-level sections. The next method of cross-section-ing in the order of complexity, and therefore in the order of. Fig. 51. accuracy, is the method of three-level sections. The area ofthe section is \{a^d){wT-^wi) — ---, which may be written II § 78. EARTH
. Railroad construction. Theory and practice . xr , This condition is fulfilled when the slope isuniform, when a = a. When this is nearly so the erroris evidently not large. On the other hand, if the slopes are in-clined in opposite directions the error may be very considerable,particularly if the angles of slope ate also large. 78. Three-level sections. The next method of cross-section-ing in the order of complexity, and therefore in the order of. Fig. 51. accuracy, is the method of three-level sections. The area ofthe section is \{a^d){wT-^wi) — ---, which may be written II § 78. EARTHWORK. 89 ^(a-^d)w —--J in which iv=Wr + itH If the volume is computed by averaging end areas, it will equal --l(^a^d)w -ab-\-\a^d)w -ah\ (59) If we divide by 27 to reduce to cubic yards, we have, whenZ = 100, Vol i,,.,. )=|t(a + ^0^-Ma64-ff(a +J)ty—|fa6For the next section Vol (,^.. _,=ff(a + d)w;-|fa&+||(a + d)^-tfa&For a partial station length compute as usual and multiply result by — -— The prismoidal correction may bo obtained by applying Eq 46 to each side in turn For the leftside we have ~A(a^d)-{a^d)\ivi-wi), which equals l-{d-d){wr-wi^. For the right side we have, similarly, l^(^d-d){Wr-Wr)^ The total correction therefore equals Reduced to cubic yards, and with Z = 100, Pris. ||(cZ-0(^-i^). • . ^ (60) When this result is compared with that given in Eq. ^^ thereis an apparent inconsistency. If two-level ground is
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