Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town

Transcription

1 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina * Luigi Colombo * Barbara Marana Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town DOI: /Ti_2_14_4e Keywords: Urban environment, Laser scanning, Documentation, Land Enhancement. Abstract Building heritage well displays the identity of the people living in different epochs: the knowledge of its original characters, and the preservation and enhancement along time, require documentary interventions, regarding both the buildings constructive features, like dimension, geo-localization and the urban shape, with others more specifically of historical-architectural or material kind. The new terrestrial laser technologies, integrated with positioning and photographic sensors, allow fast survey and spatial representation both for single building units and for whole urban districts. The large amount of 3D collected data provides an enrichment of the basic geometric knowledge, usually provided by cartographic vector or raster representation. The paper deals with a static-kinematic laser scanning test carried out over the town of Dalmine (Bergamo, Italy), according to the present municipality program of landscape safe-keeping, constructive feature preservation and improvement regarding the historical settlements dating back to the last century twenties and which are highly representative of its company-town past. The survey experiment is also meaningful to validate an operating procedure, based on new technologies, which is innovative, simple, inexpensive enough and conveniently applicable to the urban areas. THE TERRESTRIAL SURVEYING The new terrestrial surveying technologies have recently provided the diffusion of highly automated multi-function sensors which can perform a series of integrated operations, directly on the field, ranging from surveying measurement geo-referenced in a provided reference system, to a fast geometrical description of objects, achieved by single points or whole laser scanned areas, up to the overlay of photographic images, with their meaningful semantic contribution. These integrated devices can be positioned on the ground or on a vehicle, so as to perform faster and easier repositioning on different station points; on occasions, the vehicle is equipped with a retaining mechanic structure, extensible in height, so as to perform shooting also when the surveyed object has a meaningful vertical development, such as for buildings façades and building curtains. This more simple acquisition allows to enhance the survey versatility and productivity, achieving far lower costs. Starting from the 2000 s, some positioning and navigation permanent networks have been developed, together with support services provided by private companies, such as in Italy ItalPOS, managed by Leica, and NetGEO, by Geotop-Topcon, which are today largely used by professionals for geo-refe- * Geomatics group - University of Bergamo - Engineering Department, Dalmine (BG) - Italy

2 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina Luigi Colombo, Barbara Marana rencing their survey, and which have an important role for the new integrated static and kinematic survey technologies. Speaking about these systems, the recent Leica Nova MS50 total station provides an original approach based on a multi-function technology, together with the easy sharing and compatibility of the collected data. Also in the kinematic field, it is possible to get integrated mobile systems from the most well-known manufacturers and the research world has allowed to improve positioning precision (relative and absolute) in different operating conditions, to provide more reliable measurement procedures and to reduce the systematic errors due to sensor calibration and device synchronization (Habib et al., 2010). LASER SCANNING AND IMAGING The Laser scanning surveying technique provides interactive and automated features which are useful for a 3D metric integration of the classical vector and raster cartographic representation of the urban territory. It also does not acquire object detail points, according to the usual manual and deterministic survey (vector model), but it records, in an objective and stochastic way, a spatial grid of points ideally overlapping the area of interest (raster model). Therefore some interpolation techniques are required to extract the points of detail which really describe the object geometry. According to terrestrial survey, it is possible to speak as a static acquisition (of long, medium or short range), when the laser device (panoramic 3D setting) and the related photo-camera are in fixed position (both on the terrain and on a stopped vehicle), or as a kinematic one if the measurement device (profiling 2D setting) works from a mobile platform, known as Rover System. In the kinematic option, besides the laser sensors for 2D profiling and image recording, it is required a system for the vehicle geo-referencing, for the events of loss of the satellite signal (urban canyon, galleries, etc.) and for the absolute time definition (Alshawa et al., 2007). This way, the final result is a geodetic navigation system with a GNSS (Global Navigation Satellite System) receiver for positioning, global bearing and time measurement, a rigid anti-shock platform with inertial unit IMU (Inertial Movement Unit) for bearing and vehicle trajectory computing, a high precision odometer DMI (Distance Measurement Indicator) for the distance measurement and the trajectory adjustment, a time synchronization device of all the acquired information and an ad hoc management software (Britting, 2010; Groves, 2008). The differential processing of the satellite data acquired by the Rover device (when moving) requires one of the following satellite information: a) ad hoc position recordings provided by a master station or by a reference station network (in this work the data of GPS Lombardia service), with a 1 Hz sampling frequency, useful for a centimetre positioning (Post-Processing), b) interactive position corrections transmitted by a satellite master station or by a reference station network for a good real time positioning (Real-Time Processing). In the described test it has been experimented a static-kinematic laser scanning system, recently proposed by Microgeo srl from Florence (Italy); the system is light, compact and highly modular for the possibility to change its configuration (Figure 1), according to field applications and user needs.

3 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina 59 Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town 59 Figure 1 The static-kinematic system from Microgeo (Italy) THE TEST CASE PROJECT The survey project aims to acquire a spatial knowledge, to integrate the cartographic one, related to the historical areas of the town of Dalmine, regarding the fascist period and known with the name of Greppiane. These urban settlements (Figure 2) were projected, in the twenties of the former century, by the architect Giovanni Greppi and they are located around the famous Dalmine mechanic factory, nowadays Tenaris. This is one of the first meaningful examples in Italy of what nowadays would be called companytown. A detailed knowledge of this area is of great importance for the local Administration, in order to develop a program of preservation of the constructive features and valorisation of the territory buildings. The Greppi project involves the districts Leonardo da Vinci, originally built for the Dalmine employees, the Mario Garbagni district aimed at the workmen and the buildings dedicated to social, economic and administrative purposes of support to the factory, named Centro. Because of the wide extension of these three areas, it has been decided to start the survey project with a test field of about seven hectares regarding the Garbagni terraced houses. The survey of this territory has been organized, so as to provide both large urban scale (from 1:500 to 1:200) and building detail (1:100) graphical output. This way, two levels of detail (LoD, Figure 3): - a territorial framing survey, achieved with a 3D kinematic laser scanning approach and photographic texturing (imaging), to take advantage of the potentiality provided by this technology over wide areas; - a local building (exterior) survey, performed with static scanning and imaging, and extended to all the inhabited units of the district.

4 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina Luigi Colombo, Barbara Marana Figure 2 The three Greppiane zones of the town of Dalmine This project will allow later to build a spatial model of the urban area, according to the archiving and exchanging of 3D urban object model, known as CityGML (City Geography Markup Language), an OGC (Open Geospatial Consortium) standard. CityGML considers a multi-scale structure of the model, according to five following Levels of Detail, from LoD0 to LoD4, both for geometric and semantic features (fig. 4, Kolbe et al., 2008). A documentation project always starts with the setting of its purposes and of an expected level of detail, which is function of the dimension dr of the smallest recognizable object on the model with a certain probability level. The test selected values are dr = (6-15) cm for the land detail level and 3 cm for the buildings one. Employing a proper statistic relationship to these assumptions (Colombo et al., 2012; English Heritage, 2006), it is possible to find the corresponding grid step s for the points to be acquired. In both levels of detail of this test, see Figure 3, the mean grid step are s = (2-5) cm for the land kinematic survey and s = 1 cm for the static building application. Figure 3 The Garbagni land-building survey levels

5 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina 61 Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town 61 Figure 4 CityGML: the five levels of detail The laser sensors, employed for the static and kinematic test, are related to the Riegl technology, integrated with an automated NIKON camera, and are supported by the Riegl Riscan software, for data acquisition, processing and management. The Riegl sensor provide, in the established measurement range (about 50 m) a dimension b for the laser spot, over the surfaces, of about 1.8 cm and therefore well according to the best operating assumption b 2s, which relates b to the grid sampling step. Also the value of the point precision, set by the manufacturer to about 5 mm, is to the sampling step s for the two survey levels. Besides, the new Riegl technology enables full waveform analysis of the laser beam and the digitization of the repeated echo signals. Therefore it becomes possible to bypass obstacles, like vegetation and fencing, along the laser beam travel, and to record range-independent relative calibrated reflectance values, which are nearly observing range independent, related only to the surface material and then useful for a proper material analysis. STATIC AND KINEMATIC ACQUISITION The static survey of the Garbagni buildings required the choice of suitable station points, so as to acquire at best the single buildings structure, limiting as much as possible the perspective effects and providing a meaningful overlapping between adjacent scanned clouds. The shooting has been extremely influenced by the narrow spaces, the narrow position of the terraced buildings and by their fencing often lined with trees: therefore it has been planned by positioning the instrument on the local viability, trying to limit perspective acquisition which is not the best, both for the horizontal obliquity and the vertical one related to the buildings height (Lichti et al., 2006). Figure 5 shows details of the static survey for a building in the mentioned area.

6 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina Luigi Colombo, Barbara Marana Figure 5 Organization of the static survey For the kinematic survey, a positioning trajectory is established, which is a series of close station points, which creates almost a continuous acquisition process. The instrument works as a 2D device, as a profilometer, and collects a sequence of vertical sections, crossing the motion direction; the third dimension is provided by the movement of the vehicle along the selected trajectory, with the aid of satellite-inertial positioning and bearing systems. The scanning resolution, in points per m 2, of the object point model is (Rieger et al., 2010): - inversely proportional to the sensor speed; - inversely proportional to the mean distance between the vehicle and the surveyed surfaces; - directly proportional to the laser sensor acquisition frequency (khz); - independent from the number of cross section recorded per second. According to the Garbagni district morphology, it has been selected a vehicle of low mean speed, id est 20 km/h (5.56 m/s) and 100 recorded sections per second; this way, it follows, for a 20 m range between the vehicle and the building façades and a 300 khz scanner acquisition, a resolution equal to 430 points/ m 2. Since the mean distance d, among the sections along the trajectory (Figure 6a), is about 5.5 cm (longitudinal grid step), the transversal one d, among points belonging to the same section, is the about 4.5 cm (Figure 6b): the values d and d are quite close and also in agreement with the step value s = 5 cm designed for the land survey grid.

7 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina 63 Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town 63 Figure 6a Kinematic survey trajectory in the Garbagni district Figure 6b Mobile acquisition for vertical sections The integrated GNSS-IMU sensors allow a vehicle positioning with a refreshing frequency of at least 100 Hz against a (1-5) Hz sampling, proper of GNSS devices. Therefore, with a hypothetical vehicle speed of 20 km/h (5.56 m/s), the laser and photographic sensors position will be refreshed about every 5.5 cm. Thanks to this integrated technology, it has been possible to set the attitude, to correct the drift errors between subsequent measurements and providing a positioning precision up to a few centimetres, also in areas with reduced satellite reception. From a general point of view, the kinematic approach is undoubtedly convenient and productive, despite the higher processing complexity and a lesser metric quality of the acquired model, both for the detail point resolution and precision. The kinematic method directly records all the object points in a single cloud which is already geo-referenced thanks to the positioning sensors on the vehicle. The alignment step is then only to be geometrically checked, while all the steps useful to provide the geometric models need to be performed: - automatic point filtering; - undesired point deletion;

8 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina Luigi Colombo, Barbara Marana - mesh generation; - photographic texturing; - 2D plot extraction for a more effective description. For this test, for economic and time reasons, it has been decided not to use pre-signalized and measured control points over the selected buildings, even if this solution allows to check and improve the geometric quality of the reconstructed model in the kinematic approach (Barber et al., 2008). Figures 7a-b show the kinematic test results over the district building curtains and façades (Rutzinger et al., 2011; Nalani et al., 2012). From the output detail, it is easy to understand that the kinematic approach is suitable, as expected by the project, for a territorial knowledge (street layouts, infrastructures, building floor contour and relationship with the viability, volume amount, urban aggregation), but it is not yet useful at building scale documentation. Figure 7a Orthographic view of building curtains with reflectance values (kinematic survey) Figure 7b A building 3D model with reflectance values (kinematic survey)

9 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina 65 Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town 65 DATA PROCESSING The implemented software packages for kinematic and static model processing were Riegl Riscan Pro and JRC 3DReconstructor; they allow to use, in a semi-automated way, filters to clean the model points from blunders or noises, as it happens for those outside the surveyed surfaces, or acquired with too high incidence angles, or with a low return signal intensity. For the static survey, the cloud connection, in a global reference system, has been carried out with the following set of procedures. The employed Riegel sensor is provided with a GNSS positioning system, a compass and a verticality sensor. They allow all the static acquired point clouds to be plumb line oriented and already geo-referenced (position and orientation) in the world reference system, with sub-metric precision. The scan matching procedure has been refined anyway by applying the Iterative Closest Point algorithm (Besl et al., 1992) which automatically brings closer the homologous geometric primitives (points, lines or polygons) between two adjacent clouds. Current research in this filed is focused on the reduction of the propagation of the connection error with more scans and on the speed increase. This evolution leads to a multi-scan approach with a Bundle Adjustment iterative solution, which allows to simultaneously connect all the clouds, performed in this test with the package JRC 3DReconstructor. The final point model has then been transformed in a tin and in a meshed model. This procedure allows a better superimposition (texturing) of the images acquired by the camera connected to the laser sensor (with a resolution homogeneous with the cloud one), so as to allow a virtual 3D reconstruction with more thematic information (Figure 7c). Figure 7c A building 3D model with image overlay (static survey)

10 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina Luigi Colombo, Barbara Marana GRAPHIC OUTPUT The surveyed 3D models (kinematic and static) have been visualized by extracting, via software, 2D vector or raster output: for the raster output, they are mainly orthographic views, over established planes, of the territory and its buildings, with photo-textures and thematic overlays, while the vector ones are line plots of urban plans or building elevations and sections provided with CAD tools by processing the outlines acquired through cut planes on the point model (Figures 8a-b, 9a-b). With the aid of colour (RGB) and reflectance images (distance independent, thanks to Riegl sensor), it has been possible to perform thematic analyses, for instance about the building materials: these often appear with homogeneous colours in the photographic images, even if they are not, as well enhanced by the different reflectance values (Figures 10a-b). This information is meaningful for the original building feature preservation, since it is useful to identify the interventions which are not consistent with the established maintenance constraints. Figure 8a The district orthographic view, with overlay of building architectural type (kinematic survey) Figure 8b Vector output levels, with buildings, viability and green areas

11 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina 67 Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town Figure 9a Orthographic view of a building façade (static survey) Figure 9b A building elevation drawing Figure 10a Material semantic analysis (RGB values) 67

12 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina Luigi Colombo, Barbara Marana Figure 10b Material semantic analysis (reflectance values) FUTURE DEVELOPMENTS To this day the authors hope that in a short time the Communal Administration of the town of Dalmine will decide to start a new collaboration with the University, so as to complete the survey of the other historical areas and then to plan the archiving and consultation of the acquired data. A possible idea could be to store the geo-referenced vector and raster 2D output, both for the urban area and the buildings, in a database performed via the application ArcCatalog of the package ArcGIS Desktop by Esri. It could be also useful to fill in dedicated tables with a geometric-thematic description to be related to the above mentioned images. This way, it is allowed a fast and effective consultation of the district present state and also its preservation by applying constraints to the future architectural and landscape interventions. In fact, the database will be periodically updated and therefore it will provide a historical archive of everything already performed and/or planned. FINAL REMARKS The field test, performed over one of the three historical areas of Dalmine, has allowed to analyse the technological and practical possibilities of the terrestrial laser techniques, both static and kinematic, and to enhance the great evolution in the acquisition step which is more automated and fast, simpler to use and of better reliability (Maas et al., 2010). The software packages always plays a meaningful role in the production of measurable vector and raster output, which always involves huge amount of data to be processed, and for the model photographic texturing. This last step, anyway, is now more integrated with the geometric one, thanks to the possibility to acquire directly point clouds already coloured with the images (Ingersand, 2006, Reshetyuk, 2009). The main bottleneck mainly lies in the production of vector output, where manual work still plays a basic role in the level of quality provided. At Intergeo 2013, a leading conference trade fair for geodesy, geoinformation and land management, it has been recently presented a new advanced software, named Pointfuse (Arithmetica - UK), which can automatically provide a more reliable and precise way to deliver better line drawings and quality

13 Articolo 4.2 ING_Layout 1 21/01/15 12:19 Pagina 69 Terrestrial laser scanning for urban survey: a test case over the Dalmine company-town 69 vector models starting from point clouds. Anyway, it is stressed the increasing role of measurable spatial models, integrated with photographic images (Kraus, 2007), for a morphologic and semantic description of the urban landscape, till now appointed to 2D representations and to a separate management of building elevations and curtains. Acknowledgements Many thanks to Microgeo srl (Firenze) and to Diego Galbusera, Samuel Pezzotta, Giorgio Ubbiali and Pietro Azzola, students recently graduated in building engineering from the University of Bergamo. References Alshawa M., Smigiel E., Grussenmeyer P., Landes T. (2007), Integration of a terrestrial lidar on a mobile mapping platform: first experiences. 5 th International Symposium. Padua - Italy. Barber D., Mills J., Smith-Voysey S. (2008), Geometric validation of a ground-based mobile laser scanning system. ISPRS Journal of Photogrammetry and Remote Sensing 63(1). Britting K. (2010), Inertial Navigation Systems Analysis. Artech House. Besl P.J., MacKay N. (1992), A method for registration of 3-D shapes. IEEE Transactions on Pattern Analysis and Machine Intelligence. Colombo L., Marana B. (2012), Terrestrial laser scanning specifications. GEOInformatics. vol. 8. English Heritage (2006), An addendum to the metric survey specifications for English Heritage. Groves P. (2008), Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems. Artech House. Ingersand H. (2006), Metrological aspects in terrestrial laser-scanning technology. Proceedings of 3 rd IAG / 12th FIG Symposium. Baden - Germany. Habib, A., Kersting A.,Bang K.I. (2010), Comparative analysis, of different approaches for the incorporation of position and orientation information in integrated sensor orientation procedures. Proceedings of Canadian Geomatics Conference 2010 and ISPRS com. I Symposium. Calgary - Canada. Kolbe, T. H., Nagel, C., Stadler, A. (2008), CityGML - A Framework for the Representation of 3D City Models from Geometry Acquisition to full Semantic Qualification. Proceedings of ISPRS Congress Bejing, China. Kraus K. (2007), Photogrammetry: Geometry from Images and Laser Scans. De Gruyter. Lichti D.D., Licht M.G. (2006), Experiences with terrestrial laser scanner modelling and accuracy assessment. Proceedings of the ISPRS Commission V Symposium Image Engineering and Vision Metrology. Dresden - Germany. Maas H.G., Vosselman G. (2010), Airborne and Terrestrial Laser Scanning. CRC Press. Nalani N., Nirodha Perera S., Maas H.G. (2012), Automatic processing of mobile laser scanner point clouds for building façade detection. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. 39, Part B5. Reshetyuk Y. (2009), Terrestrial Laser Scanning. VDM Verlag. Rieger P., Studnicka N., Pfennigbauer M., Ullrich A. (2010), Advances in mobile laser scanning acquisition. 24 th FIG International Congress. Sydney - Australia. Rutzinger M., Höfle B., Oude Elberink S., Vosselman G. (2011), Feasibility of façade footprint extraction from mobile laser scanning data. Photogrammetrie, Fernerkundung, Geoinformation. Vol. 3(3).