短语词组短语plumb -line mean flow velocity 垂线平均流速Plumb -bob line 锤线local plumb -line variation 地方铅垂线变化direction of plumb -line 垂线方向correction for plumb -line curvature 垂线曲率改正digital plumb -line tele-coordinator 遥测垂线仪 返回 plumb-line