既有桥梁大修时,一般要封闭桥梁,在原桥旁修建临时便线维持通车。目前便线的测设方法一般均采用按新建线路的经纬仪定测办法。但一般桥梁两端河滩路堤较高,加之堤下灌木丛生,沟河水泡星罗棋布,往往给勘测工作带来很大困难。尤其在便线施工中需多次核对中线位置,每次都采用经纬仪定测将频繁地牵扯工程技术人员精力。 In the case of overhaul of a bridge, it is generally necessary to close the bridge, and to construct a temporary convenience line near the original bridge to keep it open to traffic. At present, the method of measuring and measuring the convenience lines generally adopts the theodolite measurement method according to the newly-built lines. However, the high embankment of riverbanks at both ends of the bridge generally, combined with the shrubs under the dikes and the dross blossoms, often bring great difficulties to the survey. In particular, in the construction of the mezzanine, the centerline position needs to be checked several times. Each time the theodolite is used for measurement, the engineering and technical personnel will be involved frequently.