We propose a robust calibration method for central omnidirectional cameras. It makes use of two mutually orthogonal sets of parallel lines. Based on the unified sphere model, the intrinsic parameters are initialized by two pairs of vanishing points in the image plane. Furthermore, the extrinsic parameters are estimated based on two geometric preservations of the two line sets on the viewing sphere. We test the method on both simulated and real data. The simulated experiments show the effectiveness of the proposed method. Experiments with respect to calibration results and validation of extrinsic parameters for real images prove that the proposed method has higher accuracy compared with Mei's toolbox.