隧洞测量在土坝水库各测量工种中，精度要求是比较高的，其原因如下：1．隧洞开挖工作面多是两个或两个以上，彼此同时相向开挖，这样在施工上可以争取进度，而这些工作面最后都要会合，例如密云水库隧洞共有七个开挖工作面，这样就要严密控制它的进洞方向。但这些方向线都是预先无法闭合的，单方向延伸线如有偏差，事先无法得知，所以不易作得准确。 Tunnel survey In the surveying work types of Tuba Reservoir, the accuracy requirements are relatively high. The reasons are as follows: 1. Most of the tunnel excavation face is two or more, and they are excavated towards each other at the same time. In this way, progress can be achieved in construction. These working faces will eventually meet. For example, there are seven excavation facets in the Miyun Reservoir tunnel. It is necessary to tightly control its direction of entry. However, these directional lines are incapable of being closed in advance. If there is a deviation in the unidirectional extension line, it cannot be known in advance, and therefore it is not easy to be accurate.