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A Real-time System for Road Management H. Gontran, J. Skaloud, P.-Y. Gilliéron, F. A. Bayoud Ecole Polytechnique Fédéral de Lausanne (EPFL), Geodetic Engineering Laboratory, 1015 Lausanne, Switzerland Email: herve.gontran@epfl.ch, jan.skaloud@epfl.ch, pierre-yves.gillieron@epfl.ch, fadiatef.bayoud@epfl.ch ABSTRACT Precise time registration, georeferencing and instantaneous feature extraction are important challenges for real-time mapping architectures. The Geodetic Engineering Laboratory at the EPFL has designed a real-time mobile mapping system to determine the geometry of the road via a single camera: the Photobus. This paper describes a method and experiences of distributed data preprocessing that enables Photobus to fulfill its mission in the shortest delays. In this context, image grabbing and processing is autonomously achieved by a vertically-oriented camera with an embedded PC, the Ethercam. Triggered by a GPS pulse to guarantee accurate synchronization, the vision system captures frames via CMOS pixels that are less affected by varying illuminations than their CCD counterparts. The resulting histogram-equalized images undergo feature extraction algorithms based upon binarization with an adaptive threshold and blob analysis. Implementing these image processing techniques on the camera-embedded OS helps the computer hosting the mobile mapping software to better focus upon other time-critical tasks. The second distributed system applies the concept of moving base station to provide a real-time determination of the position and heading of the Ethercam for the whole area covered by the Photobus. The proposed implementation relies on a dual-antenna GPS receiver whose primary chip permanently broadcasts RTK corrections to the secondary one, in order to obtain heading with degree-level accuracy. We designed an Internet-based regional DGPS server that contributes to the centimetre-level location of the primary GPS receiver chip on the Photobus through a GPRS connection. While gathering corrections at 5 Hz from an array of base stations, the server broadcasts RTK messages from the nearest reference. The paper describes experiences of GPRS data transfer in a production context, the effects of a changing base station and the advantages of a customized DGPS server compared with surveyor-oriented services. 1. INTRODUCTION Road databases commonly use a linear referencing system (LRS) for a spatial description of elements of interest. A LRS is directly implemented on the road, with an origin and a set of marks painted on the pavement at each kilometre. Its use does not require an absolute localization of such marks, except for cartographic purposes. Most of the GIS applications now include a procedure for the dynamic segmentation of data that are referenced either in a LRS or in a national reference system. However, the road objects tend to be added using GPS-based positioning, which requires the description of the painted marks and of the centreline geometry in both systems to find the necessary transformation. The acquisition of the needed transformation parameters initiated the design of a mobile mapping system by the Geodetic Engineering Laboratory of the Swiss Federal Institute of Technology in Lausanne. Our system can be distinguished from its predecessors by its ability to georeference the road centreline through a vertically-oriented CCD camera (Figure 1). This monoscopic technique is simple and economically appealing for rendering the road layout with sub-decimetre accuracy (Gilliéron et al., 2000).  Figure 1. The Photobus system Nevertheless, the processing of the navigation data derived from a loosely-coupled GPS/INS remains semi-automatic and still requires considerable input from a skilled operator. On the other hand, the variation of light conditions conflicts with the automation of the centreline extraction from video by deceiving most algorithms of contour detection when a part or the whole image is over-saturated. Internet-based GPS-RTK positioning and implementation of a CMOS (Complementary Metal Oxide Semiconductor) image sensor help to deal with these issues. Both solutions are discussed in the paper and contribute to the goal of achieving a near real-time mobile mapping system. This will limit the operator’s intervention to the data collection in the field while ensuring immediate quality control.
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