. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . gh country cross-sections must be inter-polated as the surface demands. As will be exj^lained later,carelessness or lack of judgment in cross-sectioning will introduceerrors of such magnitude that all refinements in the computationsare utterly 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
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . gh country cross-sections must be inter-polated as the surface demands. As will be exj^lained later,carelessness or lack of judgment in cross-sectioning will introduceerrors of such magnitude that all refinements in the computationsare utterly 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-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 accc])t- 74 BAILED AD CONSTRUCTION. 68. able this line is assumed to be strais^ht. Accordins: to the irreo^-ularitj of the ground and the accuracy desired more and more intermediate 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 from. Fig. 44. the profile or by the computed establishment of the grade distances out from the center of all breaks are determinedwith a tape. To determine the elevations for a cut, set up alevel at any convenient point so that the line of sight is higherthan any point of the cross-section, and take a rod reading onthe center point. This rod reading added to d gives the heightof the instrument (H. I.) above the roadbed. Subtracting fromH. I. tlie rod reading at any ^ break gives the height of thatpoint above the roadbed (A,, hi, h^, etc.). This is true for allcases in excavation. For fill, the rod reading at center minusd equals the II. I., which may be positive or negative. Whennegative, add to the H. I. the rod readings of the inter-mediate points to get their depths below grade ; when posi-tive, subtract the H. I. from the rod readings. The
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