High-Precision Object Delineation With Uav – Demonstrated on a Track System

Abstract

Abstract. The proper function of rail-based transport networks relies on the accurate positioning of the tracks. Regular control and maintenance intervals are in place to guarantee safe and reliable operation. This also holds for the crane rails of the storage cranes in the container terminal in the Hamburg harbour. Especially in the terminal “Altenwerder” the geomorphological conditions of the soil lead to a permanent subsidence of the tracks and thus ask for intensive surveying and maintenance activities. The allowed tolerances are in the range of 10mm in the XY-plane on a stretch of 300m. In the daily practice, the measurements are done using traditional tachymetric survey, in combination with a rail car carrying a reflector. This method is reliable but comes with the disadvantage that the operation of cranes needs to be interrupted. In this paper we present an alternative, automatic approach which employs state-of-the-art UAV-based photogrammetry to measure the actual location of the rail. The mid-format camera system combined with a 150mm tele-lens results in a GSD of 0.9mm at 35m flying height. Challenges addressed concern the proper setup and installation of the ground control network, the flight planning and bundle adjustment. Furthermore, an automated rail delineation in the derived surface model was developed. First experiments show that an automatic workflow is possible, including the delineation task. Remaining obstacles concern, for instance, the compliance with the requirements regarding absolute positional accuracy, since the inner block geometry is theoretically much more accurate than the realised control point network.

Implementation and Validation of a High Accuracy UAV-Photogrammetry Based Rail Track Inspection System

“…To achieve the required accuracy and avoid field problems, special features and requirements must be considered. These requirements were already explained in the previous paper [1] but they can be summarized as follows:…”

“…As explained in [1], in this project we are facing the difficulty that the area which is surveyed by the chosen system does not show much height variations, hence a simultaneous calibration of the internal camera parameters, especially of the focal length, is not favorable. In addition, the optics of the chosen system can only be focused to infinity from an object distance of 750 m, which is attributed to the relatively long focal length.…”

“…6 micrometers, i.e., 1.5 pixels. As described in [1], the camera was pre-calibrated in the lab of DLR (Institute of Optical Sensor Systems) applying the diffractive optical elements (DOE) method [8]. The results of [1] suggest that the final results of the bundle block adjustment do not differ significantly between different camera calibration scenarios.…”

“…In CTA3, a combination of leveled prism and coded target mounted on the top of the tripod was used in comparison to two prisms and one target used in CTA1 [1], see Figure 2. By this procedure, the number and time of the measurements from each station were reduced significantly.…”

“…The discussion focuses on the state of our system during the third field test campaign held in June 2020 (CTA3). In CTA3, different improvements have been carried out in comparison to the first campaign CTA1 [1]. For instance, a new combination of prism and coded target was employed to reduce the survey time for the ground control points (GCPs) and increase the accuracy.…”