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229 José Balsa Barreiro, Dieter Fritsch 3D Modelling of historic urban centres and… and civil engineering, as well as in certain industrial applications such as equipment maintenance, as well as for the analysis of pipelines and similar uses36. In the project carried out in Calw, the goal of this first phase was the complete acquisition of data relating to the facade of all the historic buildings of the city. For this reason, a complete sweep (360 °) was envisaged from different static positions, whose locations were previously analysed in a desk-based study. However, certain characteristics of the area studied and the eventual environment around the station at the time of scanning could influence and/or partially limit the data collection process. In fact, the appearance of noise derived from the continuous passage of people or vehicles, together with the presence of certain habitual elements in urban areas (trees, signs, etc.), hampered the equipment’s field of vision and/or degraded the quality of the scanned data. For this reason, depending on the area and the moment at which the sweep took place, the number of stations and their specific location may have turned out to be different from what had been originally planned. Good planning prior to data capture allows one to optimise resources, reduce capture times, increase volume and improve the quality of the data obtained. During this same phase, those involved must also evaluate the use and location of target boards used for the assembly of point clouds obtained from the different stations. An optimal use of available resources depends to a greater or lesser extent on the technician’s skill and familiarity with the area under study. In any case, good pre-planning of the laser scanning phase allows one to minimise and reduce the frequency of the appearance of gaps in the point clouds (figure 4). The data collected from each station was recorded in a local coordinate system, whose origin corresponds to the point where the equipment is located. For the sweeping of large areas, it is necessary to take data from several locations, which allow one to obtain different geo-referenced point clouds in local coordinate systems. Subsequently, the different point clouds must be assembled to obtain a single and complete laser point cloud of the entire studied area. With this in mind, a minimum of three target boards visible from successive stations were placed in situ, which were later used as reference points for the assembly of the different point clouds. Image 4: (a-b) point clouds obtained from scanning from a specific stationary point. Note also the presence of shaded areas due to the limitations to the laser equipment’s field of vision 36  GrindGIS, 2015. «LIDAR Data 50 Applications and Uses- It is important», GrindGIS, 18 August, 2015, http://grindgis.com/data/lidar-data-50-applications (Date accessed: 15 March, 2017). http://revista.ieee.es

REVISTA IEEE 11
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