. Railroad construction. Theory and practice . ^ wasted. 68. Cross-sectioning. The process of cross-sectioning con-sists in determining at any place the intersection by a verticalplane of the prism of earth lying between the roadbed, the sideslopes, and the natural surface. The intersection with the road-. FiG. 44. bed and side slopes gives three straight lines. The intersectionwith the natural surface is in general an irregular line. Onsmooth regular ground or when approximate results are accept-able this line is assumed to be straight. According to the irreg- § 69. EARTHWORK. 75 ularity of t
. Railroad construction. Theory and practice . ^ wasted. 68. Cross-sectioning. The process of cross-sectioning con-sists in determining at any place the intersection by a verticalplane of the prism of earth lying between the roadbed, the sideslopes, and the natural surface. The intersection with the road-. FiG. 44. bed and side slopes gives three straight lines. The intersectionwith the natural surface is in general an irregular line. Onsmooth regular ground or when approximate results are accept-able this line is assumed to be straight. According to the irreg- § 69. EARTHWORK. 75 ularity of the ground and the accurac}- desired more and moreintermediate points are taken. The distance (d in Fig. 44) of the roadbed below (or above)the natural surface at the center is known or determined fromthe profile or by the computed establishment of the grade distances out from the center of all breaks are deter-mined with a tape. To determine the elevations for a cut, setup a level at any convenient point so that the line of sight ishigher than any point of the cross-section, and take a rod read-ing on the center point. This rod reading added to d gives theheight cf the instrument (H. I.) above the roadbed. Sub-tracting from H. I. the rod reading at any break gives theheight of that point
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