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Task force 6.1.3 - Applications of Laser Scanning Technology in Deformation Measurements
The primary objective of this Task Force is to promote the use of terrestrial laser scanning as a recognised tool for spatial data capture in engineering projects. More specifically, the group will aim to advance the use of this new technology for geometric documentation and deformation monitoring in a variety of environments, particularly those at high risk and in need of remote measurements (e.g. structures, slopes, underground surveys, structural deformations of cultural heritage monuments). Further objectives are to investigate the integration of laser scanning measurements with other measuring techniques, such as conventional geodetic systems and photogrammetric techniques, and to explore the 3D modelling and visualisation issues of laser scanning data. Also, the group aims to investigate quality control and metrological aspects of the laser scanner data in order to provide recommendations for checking that the terrestrial laser scanner instrument is working correctly prior to its use as well as recommendations on its field use regarding issues such as data collection, storage, instrument independent exchange data format, use of targets for registration etc. At the time of writing this report (January 2004) the chair of the group has finalised the regular members and is creating a Task Force website which will provide a focus for terrestrial laser scanning research with links to member's websites. It will also include a comprehensive reference list for terrestrial laser scanning studies that will be regularly updated as this technology is rapidly progressing. Also, the site aims to include a number of standardised terrestrial laser data sets to allow comparison between different software and processing methods. Terrestrial Laser Scanning in Engineering Applications While three-dimensional laser scanning systems have been used for years in high precision, small-scale industrial metrology applications as well as for airborne surveys, the use of laser scanning for large-scale (i.e. greater than a few meters in horizontal range) ground-based measurement operations is still in its infancy. Little published research considers high precision, three-dimensional resolution of ground or structural movement. Several commercial terrestrial laser-imaging systems have been recently released. These have ranges of up to 350m and can acquire up to 20,000 points per second. These imaging systems provide a user with a dense set of three-dimensional vectors to unknown points relative to the scanner location. The volume of points and high sampling frequency (a full scan can be captured in few minutes) of laser scanning offers users an unprecedented density of spatial information. For this reason, there is enormous potential for use of this instrumentation in monitoring applications where such dense data sets could provide great insight into the nature of structural deformations for risk assessment, change detection and structural model validation. Two main factors influence the growth of users in engineering and surveying applications, one being the often wide gap between the commercially available scanners and the traditional surveying instruments which users are familiar with and secondly, the effective management and processing of laser scanner data. Furthermore, the emergence of laser scanning in engineering and surveying has led the need for the development of the necessary calibration protocol and the requirements for quality control assessment not only for the instruments but also for the data collection and field procedures. Some of the Current Work of Task Force Members One area where terrestrial laser scanning has been accepted as a very useful tool is in cultural heritage, as it is a natural progression from photogrammetry and the two technologies do possess many similarities. Applications vary from detailed documentation and 3D modelling to close-range structural recording (Boehler et al. 2003a, b, Barber et al. 2002, Ioannidis & Tsakiri 2003, Tsakiri et al. 2003). On the other hand, most commercially available laser scanning systems make little attempt to integrate well into existing field survey practice, although many users such as the mining industry would benefit greatly from remote surveying tools. A critical area of any new technology is the control check of the performance and metrological aspects of the instrumentation and field operation. Experiments to define the mechanical-optical stability of a number of instruments have indicated that the large weight of the currently available commercial laser scanners may be affecting a number of mechanical parameters such as eccentricity of axis (Ingensand et al. 2003). The resolution and accuracy of the distance measurement provided by different types of long-range terrestrial scanners (pulse-range or frequency type) has been the subject of study of many groups. The experiments include comparison with EDM calibrated baselines (Boehler et al. 2003c, Licthi et al. 2000a, Gordon et al. 2001b) or laboratory tests with an interferometric calibration line (Ingensand et al. 2003). Most tests indicate that the range accuracy and resolution are within manufacturers’ specifications. Further to calibration analysis, the study for the establishment of suitable test sites and control facilities for laser scanner instruments is a topic under investigation (Iavarone & Martin 2003). It is important for the test facilities to provide adequate range and dispersion of control points in order to identify range and angular errors. Also, setting the standard practices involved in the collection and archiving of data from terrestrial laser scanners is a priority area for clients and contractors alike and there is work undertaken in this area by some members of the group (Barber et al. 2003). Further advantages of the three-dimensional coordinate observations provided by the dense laser scan data sets, is that these are coupled with returned laser beam intensity. They become, therefore, radiometric data, which results in extending the scanner’s capability from a geometric sensor to a multi-spectral imaging system. Studies on spectral filtering and classification of the point clouds allow for more effective processing of the data in a spectral feature sense rather than being dependent on the spatial sampling resolution of a scanner (Licthi 2003, Lichti & Harvey 2002). The use of terrestrial laser scanning in deformation monitoring engineering applications at first may be questioned because of the relatively large single-point precision (about 5-6mm). However, the dense data sets allow for surface-wise modelling instead of point-wise analysis and provide in this way an almost ten-fold improvement in accuracy at the resultant surface model (Gordon et al. 2001a). This approach has shown that the technology can be used alike in large scale deformation applications such as in dam slope monitoring (Lichti et al. 2000b) and in small scale studies such as in precision measurements of laboratory loading tests (Gordon et al. 2002, 2003a, b). By allowing the 3D representation of a structure or testing object, the analytical models representing the bending and deforming mechanisms can be developed thus enhancing the understanding of their structural mechanisms. Benchmarking and validation of the terrestrial laser scanner data is usually performed using surveying and photogrammetric methods either in a point-wise sense or surface-wise approach. Comparison with GPS measurements (Lichti et al. 2000a) and photogrammetry-derived point coordinates (Lichti et al. 2002) has shown successful results. There is still the need to investigate rigorous methods of benchmarking the laser scanner data. Anyone wishing to participate and contribute to Task Force 6.1.5 should contact Dr. Maria Tsakiri, email: mtsakiri@central.ntua.gr and web site: http://users.ntua.gr/mtsakiri/ |
