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Authors: Pesci, A.*
Bonali, E.*
Teza, G.*
Casula, G.*
Issue Date: Nov-2010
Series/Report no.: 2010
Keywords: Terrestrial Laser Scanner
Digital 3D Modeling
Data parsing
Abstract: A time-of-flight terrestrial laser scanner (TLS) works like a laser rangefinder but is also able to scan a surface with a selected sampling step, leading to a point cloud defined into an internal reference frame. Moreover, radiometric information about signal returns intensity is provided, computed in terms of the ratio between emitted and received pulse intensities. Besides the geometric and radiometric data, some TLS instruments, equipped by an internal or an external calibrated camera, are able to provide also the RGB information by mean of a texturing procedure, i.e. a procedure of point cloud coloring. In general, the mapping of a 2D RGB image onto a digital surface model during 3D rendering, increases the realism of the resulting model. The point cloud texturing is currently used in TLS observation aimed to cultural heritage study and preservation (see e.g. Beraldin et al., 2002). The texturing procedure consists in camera calibration and orientation. The camera calibration models the true parameters of the camera (i.e. focal length, format size, principal point, and lens distortion, see e.g. Tsai 1986, 1987), whereas the orientation process leads to the position and angles of the camera with respect to the point cloud reference frame. Pairs of corresponding points, visible both in the captured RGB image and in the corresponding point cloud can be selected and used to solve for the calibration parameters of the camera in a the least square approach. Depending on the unknown quantities that must be obtained, in particular the availability of calibration parameters of the camera for the considered field, the procedure requires the detection of some or several pairs of corresponding points. In some cases, 13 parameters could be necessary, and at least 20 pair of points are therefore needed to have an adequate redundancy in the least square computation. The Optech ILRIS 3D (Optech, 2010) is a TLS instrument characterized by good performance in long range acquisition (up to 1200 m), which is equipped with an internal integrated camera (3 megapixel) to provide the RGB information and therefore allow the point cloud texturing. The format of the data directly provided by a TLS instrument is specific for the instrument. In order to allow their importation by the standard data processing software packages (e.g. Innovmetric PolyWorks, I-Site Studio, Leica Cyclone. For a list of available packages see e.g. GIM, 2009), a parsing procedure is necessary. In the case of ILRIS instrument, the parsing procedure is implemented in Optech Parser software and is well described also in the short manual provided by Pesci et al. (2009), where significant information about the planning and the definition of a reasonable acquisition protocol of a TLS measurement is also provided. A new feature available in Optech Parser allows the point cloud texturing, by means of 2D images captured at the scanning epoch by the internal camera by using the camera calibration certificate of the integrated camera. In particular, to perform the computations, the "texture image" and the “texture calibration parameter” files have to be used. The first file is the image automatically stored by the scanner and the second one is provided by Optech (Optech, inc). If these two files are directly used, the texturing procedure is completely automatic. Nevertheless, in some cases a high accuracy in RGB data treatment is necessary, and the recognition of an adequate number of pairs of corresponding point must be carried out by means of Optech Matching Viewer software package (Optech, inc). The Matching Viewer package is able to provide a texture calibration parameter file for a fine texturing. This technical report describes in detail the procedure for point cloud texturing using images from an external calibrated digital camera and Matching Viewer. In particular, the Nikon reflex D50, which is characterized by 28-mm focal length and 8.0 megapixel resolution. A TLS survey of the Santo Stefano historical square of Bologna city, aimed to provide the high resolution acquisition of Corte Isolani Palace, is performed. Figure 1 shows the external camera mounted on the ILRIS instrument during the survey. The scanner handle is shaped to allow the high precision installation of a camera on a calibrated position. Anyway the mechanical components lead to alignment errors to be corrected by means of a specific matching procedure described in the following.
Appears in Collections:04.03.09. Instruments and techniques

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