Profile-image method for measuring tunnel profile – Improvements and procedures

Fast and precise measurement of tunnel profile is needed when identifying the safety of a tunnel, especially for tunnels still under service. The profile-image method has accordingly been proposed by Wang et al. (2009). This method, utilizing a laser-lit profile taken in an image to find the 3-D coordinates of the profile, is characterized as fast in operation and precise in results. Yet, this method requires the planes of the laser-lit profile and the image in camera to be parallel and thus poses limitations on its applicability. In this study, the method is improved by parallelizing the image, instead of adjusting the orientation of camera. This paper demonstrates how the error will be significantly increased if the two planes are not parallel. The difficulty is then solved by locating all the calibration points on the periphery of a profile so that the image can be processed to make the two planes parallel. The resulted precision and the influential factors are presented. Following, the procedures of this updated profile-image method are suggested to facilitate users’ efficient employment of this method. Moreover, a field test of the updated method has been conducted. The situations that may be encountered in practice when surveying an actual tunnel, e.g. strong sunlight from the portal, are studied and the corresponding effects are discussed. Overall, it is found that the precision and operation time of this updated profile-image method are fairly good. Finally, other limitations still existing in this updated method are remarked.