Abstract: Method for calibrating a portable reference sensor system with optical sensors and at least one position sensor, comprising the method steps of a) calibrating the optical sensors of the reference sensor system to a predetermined reference coordinate system by determining a rotation matrix and/or translation matrix of each sensor so that a coordinate system of each sensor is calibrated to the reference coordinate system, wherein the respective rotation matrices and/or translation matrices are determined by detecting external calibration objects; b) calibrating the position sensor to the reference coordinate system by detecting position markers, thereby performing a calibration of a coordinate system of the position sensor to a vehicle coordinate system by determining a rotation matrix and/or translation matrix.