Master Thesis Ruxandra MOROSAN Ruxandra MOROSAN Digital Preservation of the Aurelius Church and the Hirsau Museum Complex by Means of HDS and Photogrammetric Texture Mapping Duration of the Thesis: 6 months Completion: August 2013 Tutors: Prof. Dr.-Ing. habil Dieter Fritsch, M.Sc. Wassim Moussa, M.Sc. Ali Mohammad Khosravani Examiner: Prof. Dr.-Ing. habil Dieter Fritsch 1. INTRODUCTION Fortunately in our century, the digital preservation of the heritage sites has become a topic of growing interest. Nowadays, with the help of the technique we can appeal to a series of actions to ensure perpetuate access to digital material. The representation of the 3D digital model of a heritage site has become an instrument of analysis and visualization frequently used. Urban planners as well as architects can visualize and explore different elements as the location and the size of the buildings in real time which can be of real help in their further projects. Laser scanning technology and digital close-range photogrammetry are two types of technology that can produce 3D model. Both techniques enable the collection of precise and dense 3D point clouds, the main difference between these two technologies being the different equipment that is used and the work procedure. Close range photogrammetry is a visual method where the camera orientations need to be solved first before capturing an image (Ordóñez et al., 2010). Meanwhile, like most of surveying devices, TLS need to be calibrated periodically (Santala and Joala, 2010). In this project was used Terrestrial Laser Scanner to acquire the object geometry and HD photographs have been used for having the virtual reality of the site. 1
2. DATA COLLECTION AND PRELIMINARY WORK Aurelius Church is a monastery built in the 11 th century and located in northern part of Black Forest, in Hirsau, state of Baden-Württemberg, Germany. During the War of Palatinate Succession, French troops destroyed most of the monastery buildings in 1692. This destruction left behind an impressive scenery of ruins. Up into the 1960 s archeologists were occupied with uncovering building remains and investigations on the former monastery grounds. Since 1956 the restored Church of St. Aurelius has served as a Catholic church. The Monastery Museum ( Klostermuseum) opened in 1991 on the 900 th anniversary of the church consecration of St. Peter and Paul. In a historical building adjacent to the Church of St. Aurelius, the culturally interested have since then informed themselves on 1100 years of Hirsau monastery culture and the town history of Calw. (Schloesser magazine). Taking into consideration that both the exterior and also the interior of the church must be scanned, the work was divided in two parts. Each of the parts have different requires towards the scanning equipment. For the exterior, the used instrument was Leica HDS 3000. It is a laser scanner which operates based on the time-of-flight principle, compact, fully portable and highly integrated package with digital image capture and sophisticated software tools. It also has high resolution, accuracy and dynamic survey range from 3m to 300m; the accuracy can reach the level of 4 mm. For the interior, the used scanner was Faro Focus 3D. It utilizes a constant beam of laser energy that is emitted from the scanner. Being a phase-based scanner, it measures the change of phase of the laser light allowing the scanner to calculate the distance. The FARO Focus3D is small and compact. Weighing just 5 kg (11 lb), it is the smallest 3D laser scanner ever built. With an intuitive touchscreen display, users can control all of the scanner functions with the easy-to-use interface. The survey range is from 0.6 to 120 m with distance accuracy up to 2 mm. Digital photographs have been captured by a standard calibrated digital camera NIKON D2X (12 Mpx) with 20mm lens. The images were used to texture the model. Another matter that has to be taken into consideration before performing the scanning itself is choosing the right resolution. The resolution is the distance between two subsequent measured points and determines the density of the points from the point cloud. The scanning procedure is fully automatized; it only requires pressing the scan button from the software or from the scanner. After pressing this button the scanner will proceed in collecting the points according to the predefined resolution. The period of time needed for scanning depends on the selected resolution, and the covered area and can last from 3 minutes to 120 minutes or even more for a scanner based on the time-of-flight principle. For a scanner based on phase measurements the required time to complete a 360º scan at a high resolution are only between 5 to 10 minutes. 2
3. POINT CLOUD BASED MODEL One of the standard methods to accomplish 3D models would be using terrestrial laser scanners. These devices measure in an automatic way a large number of points on the surface of an object. Each measurement records the distance to the scan station along with the horizontal and vertical angles, providing a point in space with the scanner in the origin. Because of its numerous advantages, such as very high sampling rates, the ability to gather very large amounts of data points, rapid representation of the target scene, laser scanners became nowadays very popular. In comparison with the photogrammetric method of data acquisition, this method produces a big amount of data in a short period of time. The scanning procedure consists of a series of steps from which some are automatic and others require a lot of intensive work. In order to successfully fulfill this task there are needed some fundamental knowledge about the equipment and the scanning procedure. The technique is based on the following steps: preliminary work, field operations, data registration, 3 D reconstruction, and deliverables. Registration is the process done to transform all the point clouds to a common coordinate system. 3 D reconstruction includes: data filtering, segmentation and modeling. Data filtering consists of refining the data by eliminating the unnecessary pieces of information before creating the final result. Segmentation of the point cloud has the aim to group points respect to one elements coming. Modeling is the process of creating Computer-Aided-Design objects. It is done using CAD packages. The software has special interfaces which allow the user to load the point clouds and to process those using CAD operations. Texture mapping is the last procedure that has to be made and is the procedure in which details are being added such as surface texture (a bitmap or raster image), or color to a 3D model. 4. DELIVERABLES In order for a better understanding of the real world, the geometry and semantic information have to be linked together. The modeling of an object, a natural phenomenon is a simplifycation of the real world which is based on hypothesis to be reproduced. A geographic information system is the consequence of a modeling process of the real world. A frequent used scheme in designing geographic information systems contains three hierarchic levels: the geometric, topologic and semantic levels. These three levels correspond to the different aspects which are managed by the system. There is a hard interaction between these three levels. (Mathieu Koehl and Pierre Grussenmeyer, 2000). Two of the sources that provide database for text description and 3D model are CyArk and GoogleEarth. 3
Figure 1. Part of the inside of Aurelius Church Figure 2. Integration of the 3D model in GoogleEarth 4
5. CONCLUSIONS Within the paper, a 3D reconstruction of the Aurelius Church and the Hirsau Museum Complex was performed by means of High Definition Surveying. The procedure is based on the use of laser point clouds which were manually modeled, by fitting the basic geometry shapes in the Cyclone software. Afterwards the 3D model got the final texture by wrapping the corresponding photos with the help of Google SketchUp software. The final result was delivered to CyArk foundation and submitted to Google Earth. According to the steps named below, we can draw the following basic conclusions about laser scanning technique: High speed in data acquisition 3D point measurement with high accuracy(up to millimeters) High level of automation in data processing Provides no information about surface property Laser scanning method was accurate and sufficient to produce reliable virtual 3D model 6. REFERENCES Böhm J., (2007): Orientation of image sequences in a point-based environment model. IEEE 3DIM, Sixth international conference on 3-D digital image and modeling, pp. 233-240. Moussa, W. (2010): Digital Preservation of the Hirsau Abbey by Means of Static LiDAR and HD Close Range Photogrammetry, Master Thesis, TU-Berlin, Germany (not published). Quintero, M.S. et al. (2008): Theory and practice on Terrestrial Laser Scanning, Prepared by the Learning tools for advanced three-dimensional surveying in risk awareness project (3DRiskMapping), Version 4 June 2008, pp 28-60. 5