Calibration of a Mobile Mapping Camera System with Photogrammetric Methods

The road network is an essential economic factor for the German industry. Until now, the whole horizontal and vertical alignment parameters are not known. To determine these parameters the Technische Universität Dresden has implemented a measurement system which is realised in a mobile mapping vehicle. The car includes the following sensors: Position and orientation system Applanix POS LV 420, radar sensor, eye movement gaze detection and stereo cameras. The two cameras record the road parameters as well as traffic signs with high accuracy. To assure a high accuracy over time it is necessary to calibrate the stereoscopic camera system. The stationary calibration is carried out on a self-made calibration field with thirty control points. A retro reflecting marking material allows an easy and robust detecting and measuring even without additional light sources. The vector from vehicle’s origin to the principle points of the two cameras can be calculated based on the local calibration and the UTM coordinates of the control points. Finally, this vector and the IMU angles define the orientation of the two cameras during the measurement drive. During operational use, the stationary calibration is not practicable. Mainly a new calibration is necessary if one of the cameras has to be detached during operation. The reference bar calibration is a technique where a bar with a known length is moved in object space. The targets will be detected with a matching algorithm, followed by an adjustment to determine the centres of those with subpixel accuracy. A self-calibration bundle adjustment is used to evaluate the relative orientation parameters. On every position, the constant length of the reference bar describes additional restriction equations. These restrictions stabilise the whole adjustment and should allow a complete calibration of both cameras, including the interior orientation parameters.