Imaging systems require to be calibrated. The geometric calibration consists in estimating several parameters describing the projection geometry. Just like in computer vision for cameras, intrinsic parameters characterize the internal parameters of X-ray projection system. The extrinsic parameters define the orientation and position of the acquisition system. In X-ray computed tomography (CT), the acquisition systems are generally composed of a detector and a X-ray source. The object to be reconstructed lies in-between. A perfect knowledge of the calibration parameters is needed for the reconstruction algorithm to reduce artefacts. In this paper, we focus on off-line calibration methods for 1D linear X-ray detector systems. We first introduce a calibration method for systems composed of a single linear detector. This method solves the problem of calibration in two steps using calibration objects based on four co-planar lines. Moreover, we generalize the single linear detector geometric calibration method to a multi-linear detector system. We compare four different numerical models and methods. Three are based on non-linear equation systems. Finally, we propose an adaptative calibration object.