. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . istance from what may be called the backbone line ofthe survey. 11. Cross-section method. This is the only feasible methodin a wooded country, and is employed by many for all kindsof country. The backbone line is surveyed either by observ-ing magnetic bearings with a compass or by carrying forwardabsolute azimuths with a transit. The compass method hasthe disadvantages of limited accuracy and the possibihty ofconside
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . istance from what may be called the backbone line ofthe survey. 11. Cross-section method. This is the only feasible methodin a wooded country, and is employed by many for all kindsof country. The backbone line is surveyed either by observ-ing magnetic bearings with a compass or by carrying forwardabsolute azimuths with a transit. The compass method hasthe disadvantages of limited accuracy and the possibihty ofconsiderable local error owing to local attraction. On the otherhand there are the advantages of greater simplicity, no necessityfor a back rodman, and the fact that the errors are purelylocal and not cumulative, and may be so limited, with care, that §11. KAILEOAD SURVEYS. 15 they will cause no vital error in the subsequent location transit method is essentially more accurate, but is liableto be more laborious and troublesome. If a large tree is en-countered, either it must be cut down or a troublesome opera-tion of offsetting must be used. If the compass is employed. Fig. 4. under these circumstances, it noed only be set up on the far sideof the tree and the former bearing produced. An error inreading a transit azimuth will be carried on throughout thesurvey. An error of enly five minutes of arc will cause an oft-set of nearly eight feet in a mile. Large azimuth errors may,however, be avoided by immediately checking each new azimuth id EAlLROAB CONSTHtJCTlON. | 12. with a needle reading. It is advisable to obtain true azimuthat the beginning of the survey by an observation on the sun* orPolaris, and to check the azimuths every few miles by azimuthobservations. Distances along the backbone line should bemeasured with a chain or steel tape and stakes set every 100feet. When a course ends at a substation, as is usually the case,the remaining portion of the 100 feet should b
Size: 1326px × 1885px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorwebbwalt, bookcentury1900, bookdecade1920, bookyear1922
