. The elements of railroad engineering . -brush, but one additional base line can be used. It shouldbe about midway between the extremities of the principalbase line and at right angles to it. If, however, the groundis open with nothing to interfere with long sights, two baselines should be laid out, one at either extremity of the mainbase and at right angles to it. 1342. Measurements.—All measurements should bemade with a standard steel tape and plumb-bob and care-fully checked. The base lines especially should be measuredwith great care, as the correctness of all the subsequentmeasurements d
. The elements of railroad engineering . -brush, but one additional base line can be used. It shouldbe about midway between the extremities of the principalbase line and at right angles to it. If, however, the groundis open with nothing to interfere with long sights, two baselines should be laid out, one at either extremity of the mainbase and at right angles to it. 1342. Measurements.—All measurements should bemade with a standard steel tape and plumb-bob and care-fully checked. The base lines especially should be measuredwith great care, as the correctness of all the subsequentmeasurements depends upon the degree of accuracy withwhich these primary lines are measured. 1343. Base Lines and Subdivisions.—The rect-angular method of surveying town sites is illustrated inFig. 327, in which A B is the principal base, and the aux-iliary bases A D and B Tare laid off from the extremitiesof A B. The avenues are at right angles to A />, and the LAND SURVEYING. 737 streets parallel to A B. Avenue A is parallel to the railroad,. from which it is separated by a 25-foot alleyway and onetier of lots 125 feet deep. Avenues are 100 feet in width, 738 LAND SURVEYING. streets 60; blocks 250 feet by GOO feet, fronting 250 feet onthe avenues, and the lots are 25 feet by 125 feet. The initial point A of the principal base A B is the centerof an avenue, and should be fixed by a plug 2 X 2 X 18*driven flush with the surface of the ground and the centermarked by a tack, with a guard stake beside it, and num-bered 0. Drive a temporary plug at X to be used as a fore-sight in giving the direction of A B. Set up the instru-ment at A and sight to A, frequently checking the from A on A B 50 feet, and drive a 12 plug, care-fully centering the same. This point marks the north sideof Avenue B. Continue measuring on the line A i>, drivinga stake at each hundred feet, marking the exact measure-ment by a tack, and number in regular succession from 650 feet from A set
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectrailroadengineering
