1 A moving 3 D laser scanner for automated 2 underbridge inspection 3

Recent researches proved that the underbridge geometry can be reconstructed by 11 mounting a 3D laser scanner on a motorized cart travelling on a walkway located under the bridge. 12 The walkway is moved by a truck and the accuracy of the bridge model depends on the accuracy of 13 the trajectory of the scanning head with respect to a fixed reference system. In this paper, we 14 describe the metrological characterization of a method that uses non-contact systems to identify the 15 relative motion of the cart with respect to the walkway; the orientation of the walkway with respect 16 to the bridge is determined using inclinometers and optical rails, while the position of the truck with 17 respect to the bridge is measured using a conventional odometer. The measurement uncertainty 18 of the proposed system was initially evaluated by numerical simulations and successively verified 19 by experiments in laboratory conditions. The complete system has then been tested in operative 20 conditions; the validity of the proposed approach has been demonstrated by comparing the 21 geometry of buildings reconstructed with the proposed system with the geometry obtained with a 22 static scan. Results evidenced that the errors are approximately 6 mm. 23