For surveillance applications of wireless sensor networks, analysis of sensing coverage and quality of sensing is crucial. For rough terrains where obstacles block the sensing capability, region-based approaches must be employed to determine the sensing quality. In this paper, we present a method to determine the breach paths and the deployment quality defined as the minimum of the maximum detection probabilities on the breach paths in the presence of obstacles. We propose the utilization of watershed segmentation on the iso-sensing map that reveals the equally-sensed regions of the field-of-interest in a surveillance application. Probabilistic sensor models are utilized to produce the iso-sensing map considering the sensing coverage degree and reliability level as the design criteria. The watershed segmentation algorithm is applied on the iso-sensing map to identify the possible breach paths. An algorithm is proposed to convert the watershed segmentation to an auxiliary graph which is then employed to determine the deployment quality measure (DQM). The effects of the sensor count and coverage degree on the DQM are analyzed.@en