3D Laser Scanning for the Documentation of Cave Environments

3D Laser Scanning for the Documentation of Cave Environments Maria Tsakiri, Kostas Sigizis, Harilaos Billiris, Stefanos Dogouris School of Surveying Engineering, National Technical University of Athens, Greece ABSTRACT Historically the main methods of communicating the appearance and shape of a cave are cave surveying and cave photography. To-date, the recent emergence of terrestrial laser scanning has enabled the 3D documentation of underground environments in a faster and reliable way. Laser scanners provide users directly with a three-dimensional sampled representation of an object or surface and are used in a diverse range of applications requiring recording and documentation. Their high resolution, accurate and rapid capture capabilities with rates that exceed several thousand points per second allow cost effective production of surface models for geometric documentation and visualisation purposes. This paper discusses the use of terrestrial laser scanning for the documentation of a cave site. Specifically, the cave is located at the Greek island of Kalymnos and has a rich decoration of stalagmites and stalactites. The data collection is described and emphasis is given on the processing of the 3D laser data for the construction of a model that gives the geometric representation of a complex structure with unknown topological type and sharp features. 1. INTRODUCTION Caves are a significant natural resource as are often conduits for groundwater and harbour species of wildlife. In addition, some caves are important archaeological or historic sites that provide essential information about geology, or prehistoric life. Also, they provide opportunities for recreation, exploration, and scientific study. Caves are a special kind of wilderness and an important part of a country s natural heritage. For the documentation and mapping of caves it is useful to implement systematic methods which can depict and represent in three dimensions the orientation and extent of a subsurface void. Also, when the goal is to preserve and share a record of the geometry and appearance of caves, then explicit shape information must be acquired and stored in an adequate representation that also includes some form of appearance modelling. The main methods of recording caves are drawing, surveying and photogrammetry. The simplest of all three methods is free-hand drawing but requires extensive field time. The use of grid squares over the object allows transferring more systematically the details but many inaccuracies are present in the final drawing, which by no means is a true geometric record of the cave. The implementation of surveying methods provides accuracy by using special instrumentation and measuring techniques. The simplest approach uses compass, tapes and inclinometers but the resulted map suffers from geometric inaccuracies unless very careful and cumbersome measurements are obtained. However, the location inside rock or underground and the conditions of dampness and uneven ground make ground survey methods impossible. Equipment such as total stations provides the highest accuracy by determining the three-dimensional coordinates of selected points of the cave. The operation of this equipment is based on measuring angles by means of electro-optical scanning of extremely precise digital bar-codes and distances by modulated microwave or infrared carrier signals. The points are selected in such a way to form a digital terrain model of the cave and their coordinates 403

in a specific coordinate reference system are input in any CAD software package to create the map of the cave. Although the resulted map is of high precision, the number of points acquired by total station surveying is limited because of the arbitrary selection that must be performed by the surveyor. However, total stations are always required to establish the control network for any subsequent work. Photogrammetric surveying, being a mature technology, has been extensively used for the documentation of cultural heritage monuments. In traditional approach, stereo photographs of the object are taken with a metric camera and stereo model restitution follows. Using digital close range photogrammetric methods, the products are digital orthophotos which are very useful in cave recording. However, there are many factors that may restrict digital photogrammetry from being implemented as the main technique for cave recording. The data capture can be hampered by the conditions of darkness and dampness in the cave. There are difficulties in the data processing related to the requirement of suitable points for image correlations, yet the walls of the cave do not usually posses a surface texture suitable for this process. Also the image correlation algorithms may fail due to the difficulty in generating an accurate digital elevation model (DEM) of the curved surface walls. Digital photogrammetry has been successfully used in recording cave painting rather than documenting a whole cave (e.g. Chandler et al, 2005; Ogleby 1999). An alternative and state-of-the-art method suitable for recording underground environments and caves is the use of terrestrial laser scanners. High resolution laser scanning is a relatively new technology that offers unprecedented levels of data density for close range applications. The scanner s pulsed laser ranging device coupled with beam deflection mechanisms facilitates rapid acquisition of literally millions of three-dimensional point measurements. Perhaps the greatest advantage of such a system lies in the high sample density that permits accurate and detailed surface reconstruction and modelling as well as superior visualisation relative to existing measurement technologies. In light of this notion, the geometric recording of a cave using terrestrial laser scanning technology is presented in this paper. Section 2 provides a brief description of terrestrial laser scanning and how this has been implemented for the data collection of a cave located at the Greek island of Kalymnos. Section 3 considers the processing aspects of the acquired data, discusses the visualisation capabilities of the generated products and gives results of the modelled surface. The conclusions from this work are presented in Section 4. 2. TERRESTRIAL LASER SCANNERS FOR CAVE RECORDING 2.1 Terrestrial laser scanners Ground-based laser scanning is an emerging technology offering great potential for rapid collection of dense, three-dimensional spatial data sets of entire surfaces. Commercial ground-based laser scanners measure range by the pulse method while other methods include continuous-wave ranging and triangulation (e.g. Boehler & Marbs, 2002). To measure by the pulse method, the scanner emits a brief pulse of laser light, which, after reflection by the object being measured, is sensed by a photodetector. Several ground-based laser-scanning systems are currently available. These vary in construction, operation, maximum range, accuracy, resolution and central laser wavelength. Systems also vary in size, with new generations of the hardware becoming progressively more compact. Most allow interactive selection of the scan area through digital imagery captured with an on-board video camera. Some systems have levelling, forced centring and scanner orientation establishment capabilities. All systems are driven by proprietary software. The scanner observables are range, elevation angle (or, equivalently, zenith angle), and horizontal direction, which are made relative to the scanner s internally defined coordinate system (scanner space). Cartesian coordinates, (x, y, z), are the quantities provided as output from most scanner software packages and are usually treated as observables, the so-called scanner point clouds. In the majority of applications, there are more than one point clouds required to cover the entire surface of an 404

object and the merging or registration of all acquired scans at a common coordinate system is called georeferencing. The georeferencing problem in the laser-scanning context involves transforming the observables from the scanner space to the object space. The advantage of terrestrial laser scanning is that, although individual sampled points are low in precision (e.g. ±2mm to ±50mm), converting a measured point cloud into a realistic 3D polygonal model is effective for describing the shape of a structure and can satisfy high modelling and visualization demands. While surface reconstruction is a well studied problem in computer graphics the issues for surface reconstruction and modelling of closed surfaces from dense laser scanner data are receiving great attention as they are not completely solved. The main goal behind all surface reconstruction algorithms given a set of sample points P that are assumed to lie on or near an unknown surface S is to create a surface model S' approximating S. A surface reconstruction procedure cannot guarantee the recovering of S exactly, because the information about S is only available through a finite set of sample points (Fabio, 2003). For example, when the laser scanner data do not satisfy certain properties required by the algorithm (e.g. good distribution and density, low noise), the reconstruction algorithm produces incorrect results. Therefore, the correct reconstruction method depends on the application requirements. Despite of the reconstruction algorithm being implemented, the conversion of the measured data into a consistent polygonal surface follows four stages: preprocessing, where erroneous data are eliminated; determination of the global topology of the object's surface, where the neighbourhood relations between adjacent parts of the surface has to be derived; generation of the polygonal surface, where triangular meshes are created satisfying certain quality requirements; and post-processing, where editing operations are applied to refine and perfect the polygonal surface. The use of terrestrial laser scanning in cave recording has been noted in the form of documenting rock art (e.g. El-Hakim et al, 2004). Caprioli et al. (2003) report on the pilot use of terrestrial laser scanning at caves belonging to a speleological complex in Bari, Italy but only a limited number of scans were obtained and therefore, a surface model could not be constructed. Valzano et al. (2005) have demonstrated that for the 3D documentation and virtual restoration of a byzantine crypte the use of terrestrial laser scanning proved a powerful tool. However, it was shown that the creation of models of large objects and structures required the combination of a number of techniques. Although terrestrial laser scanning has been recently used for cave recording, it is clearly the most promising tool for this type of environments. 2.2 Recording the Kefalas Cave The Kefalas cave is located at the southwest part of the greek island of Kalymnos. The cave comprises a main area of about 100m 2 and smaller passages, all decorated with some notable formations of stalactites and stalagmites. The cave in its current state is not offered to the public for recreational visits but it is in the interest of the local authorities to develop this, hence the need of recording. The data acquisition was performed using the terrestrial laser scanner iqsun 880HE80 (www.iqsun.com) and the total station TCR303 (www.leica.com). The specific laser scanner has a 360 field of view and acquires over 240,000 points per second with accuracy of less than 3 mm at 30 m range. Initially, using the total station a network of 14 control points was established inside the cave. The first point of the network located at the cave entrance to transfer reference and its coordinates at the Greek reference geodetic system were provided by differential GPS. The misclosure of the traverse was in the order of 0.07 grads for the angles and 0.002m for the distances. To minimise the amount of time spent in the cave, a strategy for scanning was defined well before starting the work. Eight setups of the scanner were used to record the main areas of the cave. The remaining passages of the cave were extremely narrow to permit the use of any instrument. To enable the registration of the overlapping scans in a common coordinate system, special spherical targets of 15cm diameter were used during each scan. The coordinates of each target were calculated and then used in the registration process to allow merging of all scans in a common point cloud (Figure 1). Each scan had duration of 405

less than one minute to capture about 800,000 points at a density of 2.5cm. The total duration of the field work was two days. Fig. 1. Extract from point cloud showing cave wall and the spherical target. 3. DATA PROCESSING AND RESULTS Terrestrial laser scanner technology produces point clouds of very high density in a very short time. When surveying an object of complex geometry, multiple scans are typically required to generate an occlusion-free 3D model. The principal task associated with building 3D models from laser scanner data in such cases is to transform the scan data from the sensor's local coordinate system into a uniform Cartesian reference datum. This is necessary in order to join the multiple point clouds by implementing the so-called registration process. This process is enabled through the use of the special targets (spheres) that were scanned during the actual data acquisition. However, in many cases the number of laser scanner points describing the sphere surface was not adequate for the proprietary software to recognise these targets automatically (cf. Figure 1). Therefore, dedicated Matlab code was developed to define the centre of each sphere and then input these coordinates in the registration software to enable the registration process. Also, due to the limited number of the special spheres during data acquisition, the registration was finally achieved at a suboptimal level (3.5 cm) but was adequate for the requirements of the cave mapping. The registration process was performed in the Cyclone software (www.leica.com). The following stage of processing is the reconstruction procedure. The core part of this procedure is the triangulation which converts the given set of points into a consistent polygonal model (mesh). This is a sophisticated and resource intensive process, even for specialised 3D modelling software and is currently under research investigation. In this project the triangulation process was performed within the Geomagic Studio software suite (www.geomagic.com). There are three basic modules in this software, namely point, polygon, and shape. On point phase, editing of point clouds is performed while the polygon phase carries out the triangulation process and, finally the shape phase executes the modelling process using NURBS (Non Uniform Rational Basis which is the process of mathematically exact representation of freeform surfaces). During the point phase, a number of manual operations were executed in order to have the registered complete point cloud cleared from noisy and erroneous points. The initial cloud of over 6 million points was further decimated because this large data volume was requiring exceptionally high computer capacity. The corrected point-cloud was converted into polygon model (mesh) using triangulation algorithms. The model of the main cave hall comprised over 12 million triangles. However, the triangulation process usually results in a model that contains a number of gaps, holes, slivers, and overlaps. This is extremely common in 3D modelling, and there are a variety of methods 406

used in filling these. Even when a surface is scanned repeatedly from different angles, occlusions may prevent access to the deepest crevices. Surfaces with complex shapes and multiple boundary components are commonly faced with the problem of holes. Holes can also be caused by low reflectance, constraints on scanner placement, or simply missing views (Figure 2). In Figure 2, part of the created polygon model which contains a large number of holes due to the occlusions during data acquisition, is seen. The surface of the cave wall is clearly seen with the stalactites formations. However, the number of gaps and holes in the data are too many for creating a smooth and water-tight model and processing operations were performed to optimise the surface model. Fig. 2. Initial model. Fig. 3. Model with major holes filled. Davies et al (2002) categorise the algorithms for hole filling in two types. The first type of algorithms are applied in an already constructed surface aiming to fill each hole with a patch that has a topology of a disc, and the second type of algorithms are applied in the point-clouds and treat the union of all the scans as an unorganized set of 3D points to be fit with a continuous surface. While the above techniques are applicable mainly in cases that require a watertight surface that bounds a volume of space, for example fabrication of physical replicas, there are other applications whereby a surface reconstruction method that preserves the geometry where it exists and smoothly transitions to plausible geometry in unobserved areas is more appropriate. An uncomplicated solution to bridging holes can be the use of external data in the form of either points or patches which are integrated in the triangulation process in order to connect the surface boundaries. In the cave case, there were no external data points available to be used in the bridging process and the software was allowed to construct patches to fill each hole. In the cave case, the hole bridging process was performed using patches but in situations of large gaps in the data this practise caused errors in the surface model. Considering that there were no external data to provide further geometric information for surface reconstruction, it was decided to allow holes in the final model rather than producing erroneous surfaces (Figure 3). A number of models with different levels of complexity were created from the original data: one 12 million-polygonal un-textured model (10 mm resolution) of the cave, a 10 million-polygon of the main hole and most holes filled and, a lighter textured model with 5 million polygons. From the model of the main hall, geometric information such as sections and cross- sections were also produced. For a rendered product, the texture of the final model can be enhanced with information from photographs taken from the internal CCD camera of the scanner; however the iqsun 880HE80 laser scanner does not have such capability. Photographs taken externally could also be used but it requires careful planning for obtaining a sufficient number of photographs from known points and in known directions within the cave so a complete rendered geometric representation of the interior surface can be built up by stitching together the frames onto the model. The rendered model can then be used for a true walk-through experience whereby a number of positions need to be chosen throughout the cave complex and be marked on a cave plan so that the software can join up the views intelligently later. But even with the produced un-rendered model of the cave, a virtual walk-through was created within commercial visualisation software packages. 407

4. CONCLUDING REMARKS The ability to create a digital and geometrically true representation of the shape and appearance of an underground environment finds very challenging applications in high-resolution recording of heritagerelated sites such as caves. To-date, cave recording and mapping expands beyond the traditional dependence on 2D representations of plans, elevations, and sections by using high-resolution recording techniques, such as photogrammetry and terrestrial laser scanning. As within any industry, however, the rate of adoption of terrestrial laser scanning as a new technology is a function of equipment costs, available skills needed to implement the new technology, and modifications to operations. It has shown in this paper that terrestrial laser scanning is a very appealing tool that allows in a fast and efficient way the 3D model building as a commercially available solution to this so-called as-built documentation. The wealth of data provided by a laser scanner allow for the construction of 3D models, a directly perceivable three-dimensional representation of the appearance of a complex cave structure. Τhe models contain information that can be examined and enhanced; for example, features or fine details that are small or only visible from a distance can be interactively analysed. A great advantage is that caves or underground sites that are closed for conservation reasons can still be studied and visited once a 3D virtual model has been created. The potential of modelling as-built reality opens new avenues in heritage applications such as virtual restoration or virtualised reality tours. Models of large objects, structures and environments are possible but as demonstrated here when limiting acquisition time is critical and a large field of view has to be used then surface details may not be captured without the combination of other techniques but the overall shape of the cave will be excellent. It was also addressed in this paper that the creation of a 3D model production from laser scanning point clouds is a relatively automated procedure, while there are also techniques for automatic filling of holes provided by the great availability of commercial software packages. However, it is required to have knowledge of the reality because the non judicious use of hole-filling tools may introduce erroneous surfaces into the final model. ACKNOWLEDGMENTS The authors acknowledge the assistance of Mr Jochen Franke from Curtin University of Technology, Australia in the data collection with the iqsun 880HE80 scanner system. REFERENCES Boehler, W., and Marbs, A., 2002. 3D Scanning instruments. Proc. CIPA WG 6 International Workshop on Scanning Cultural Heritage Recording, 1-2 September, Corfu, Greece. pp. 39-44. Caprioli M., Minchilli, M., Scognamiglio A., Strisciuglio, G., 2003. Architectural and natural heritage: virtual reality with photogrammetry and laser scanning. Proceedings of XIXth Int. Symposium CIPA, Sept 30-Oct 4, Antalya, Turkey. Chandler, J.H., Fryer, J.G. and Jack, A., 2005. Metric capabilities of low-cost digital cameras for close range surface measurement. Photogrammetric Record, 20(109):12-26. Davies J., Marschner S., Garr M., Levoy M., 2002. Filling holes in complex surfaces using volumetric diffusion In: Proc. 1st International Symposium on 3D Data Processing, Visualization, and Transmission, June 19-21, Padua, Italy. El-Hakim, S.F., Fryer, J.G. and Picard, M., 2004. Modelling and visualisation of aboriginal rock art in the Baiame cave. Proceedings ISPRS Int. Archives of Photogrammetry and Remote Sensing, 35(5):990-995. Fabio, R., 2003. From point cloud to surface: the modelling and visualisation problem. International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXIV-5/W10. Ogleby, C., 1999. From rubble to virtual reality: photogrammetry and the virtual world of ancient Ayuthaya, Thailand. Photogrammetric Record, 16(94): 651-670. 408