. Railroad construction. Theory and practice . profitableinvestment. PRELIMINARY SUR\^YS. 10. Character of survey. A preliminary railroad survey isproperly a topographical survey of a belt of country which hasbeen selected during the reconnoissance and within which it isestimated that the located line will lie. The width of this beltwill depend on the character of the country. When a railroadis to follow a river having very steep banks the choice of loca-tion is sometimes limited at places to a very few feet of width 10 RAILROAD CONSTRUCTION, § n and the belt to be surveyed may be correspondin
. Railroad construction. Theory and practice . profitableinvestment. PRELIMINARY SUR\^YS. 10. Character of survey. A preliminary railroad survey isproperly a topographical survey of a belt of country which hasbeen selected during the reconnoissance and within which it isestimated that the located line will lie. The width of this beltwill depend on the character of the country. When a railroadis to follow a river having very steep banks the choice of loca-tion is sometimes limited at places to a very few feet of width 10 RAILROAD CONSTRUCTION, § n and the belt to be surveyed may be correspondingly very flat country the desired width may be only limited by theability to survey points with sufficient accuracy at a considerabledistance from what may be called the ^backbone line of thesurvey. II. 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 forward. Fig. 4. absolute azimuths w^ith a transit. The compass method nasthe disadvantages of limited accuracy and the possibility ofconsiderable local error owing to local attraction. On the other § 12. RAILROAD SURVEYS. 11 hand 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, thatthey 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 employedunder these circumstances, it need 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 only five minutes of arc will cause an o
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