This paper addresses the NP-hard problem of deploying wireless sensor networks on 3D terrains. On the contrary to previous works that place the sensors without any analysis of the terrain, we propose a two-phase solution based on terrain partitioning. The main idea is to estimate the number of sensors to be used and simplify the sensors deployment by partitioning the terrain according to topographic criteria. Simulation results based on real-world terrains confirm the efficiency of our solution in terms of coverage quality.