摘要：The Qinling tunnel with length of 18. 488km is located on the railway line from Xi’an (Shanxi Province) to Ankan (Sichuan Province) in the middle of Qinling mountain. It is the longest double track railway tunnel in China and takes the third place in the world. According to the design, the break-through error in vertical direction caused by the altimetric control surveying is limited to 18mm for the case of one piercing face. Because the leveling route reaches over 120km in length and must go over two mountains in 2 800m height, the first-order precise leveling and precise gravity measurement should be carried out in the construction stage. In this paper the field leveling approach, the application of new technology, some experience as well as the office calculation with final results analysis are introduced. By meticulous planning, organization and observation, the final accuracy of vertical difference between two tunnel end points is only 8mm, and it provides reliable surveying guarantee for this great tunnel engineering. Finally it is pointed out that this vertical difference distinguishes obviously from the primary measurement result as high as 114mm. It means that the primary result is not accurate enough.
关键词：tunnel; break-through error; vertical control surveying; gravity measurement
