Success Stories EU Study VIGIE 2020/654 Cologne Cathedral Cologne Cathedral, Cologne, Germany Coordinator: Douglas Pritchard Organizations/Companies: Metropolitankapitel der Hohen Domkirche Köln Dombauhütte Zoller + Frohlich GmbH, Germany Hochschule Fresenius, Germany Described as an 'exceptional work of human creative genius' (UNESCO 1995), the Cathedral was granted UNESCO World Heritage status in 1996. The iconic building is of tremendous emotive value to the citizens of the Cologne and the German nation; it is also considered one of the most significant architectural structures of European Christianity. The design of the 157-metre twin tower structure dates back to 1164, and during a brief period from 1880 until 1890, it was considered the tallest building in the world. Throughout its long history, though, the Cathedral has endured numerous challenges and threats. Neglect, war, vandalism, urban growth and environmental pollution have all had a significant impact on the physical fabric of the building. Current conservation challenges include the impact of adjacent engineering works, regional seismic activity, air pollution accelerating stone decay, global warming, vandalism and excessive tourism. EU VIGIE2020/654 Study at the request of and financed by the European Commission, Directorate-General of Communications Networks, Content & Technology 12 Success Stories EU Study VIGIE 2020/654 Description Initiated as a research project with the Dombauhütte and Hochschule Fresenius Köln, the original objective was to demonstrate the effectiveness of contemporary terrestrial systems (TLS) at a complex, dimensionally-challenging heritage site. Despite the prominence of the building, remarkably at the start of the project, the Cathedral had never been completely laser scanned. As confidence in the project grew, the scope expanded to include the documentation of the entire World Heritage Site, the building exterior (at various parapet levels), the roof areas and the two immense towers. The project also included all of the central (nave, crossing, choir, side aisles, transept, etc.) and secondary spaces (crypt, apse, sculpture storage space, roof areas, etc.) within the building. In discussion with the Dombauhütte, it was agreed that TLS systems would provide dimensional point data with insufficient quality and resolution (Böhler 2003) for their conservation and maintenance purposes. Further justification for the use of TLS included: The Cathedral has been damaged, repaired and modified over the centuries and existing records may be inaccurate or under-detailed; A High Gothic cathedral is exceptionally ornate and difficult to document using traditional survey methods; Despite the Cathedral's considerable size, due to the numerous parapets and ledges that surround the building, the distance between scanner and building surface is well within the range of a phase-based system; As recognised by UNESCO, the building has profound international significance and deserving of a comprehensive, detailed archival record. During the initial planning stage of the project the location of the scanners was based on the requirement to have the overall architectural form of the Cathedral captured, but importantly, as much exterior and interior surface area coverage as possible. As an archival dataset, the adjacent architectural context such as the train station plaza and museum precinct was also included. The TLS position points were based on several factors such as ideal laser range, data resolution, data overlap, areas of possible occlusion and visual obstruction. Specific challenges included the highly articulated facade, the numerous flying buttresses and spires, the various parapet levels, and the height of the two towers. Hardware Selection The two scanning systems used in the project were the Zoller + Frohlich (Z+F) Imager 5010C and the Imager 5010X terrestrial laser scanners. Both systems were ideal systems for the Cathedral project due to their speed, range, data resolution, unit weight and critically, the integrated High Definition Range (HDR) camera system. The Z+F 5010C and 5010X are phase-based systems, utilising a Class 1 laser, which is invisible, and rated as harmless, completely eye-safe for both operator and public. A concern when scanning with substantial tourist activity at a World Heritage site. Relative to other contemporary scanning systems, the 5010C and 5010X have an exceptionally high data acquisition rate of .06 million-dimension points per second. The systems had a range of 187 meters, capable of acquiring one million-dimension points per second while maintaining accuracy above 1 millimetre (with the scanner at 20 meters from the surface). The scanners generate 360-degree point data based on a local coordinate system with intensity values. On completion of the scanning sequence, the scanners initiated a series of 42 individual images; these are then combined to form a single 80-megapixel image. An essential consideration in the documentation process is that the exact nodal point of the internal 5010C and 5010CX laser sensors and the onboard camera CCD sensor are at the same position. The result is that during the data post-processing, the imagery sits precisely onto the point data, providing a photorealistic scan dataset. For most of the interior documentation, a Z+F SmartLight was attached to the 5010C to assist in the illumination of dark surfaces. The 1000 lumen light is specifically designed for the scanner's onboard camera with a neutral colour temperature with a range of 1-10 meters. Although the 5010C has an HDR camera and capable of balancing low and high light situations, a supplementary lighting system provided higher detailed colour imagery and helped compensate for limited lighting conditions. The additional light was particularly useful when scanning the nine baroque 'Triumph of the Eucharist' tapestries suspended along the nave. Planning One of the more compelling features of a Gothic cathedral is the explicit architectural order and continuity of structural features. The positioning of the laser scanners during the project was purposely based on working with this existing architectural order. For example, the scanner was located at the centre of an interior bay between the columns. Or, at specific points between the buttresses or spires. The rationale was that these positions provided the best view for the scanner, but given the size and complexity of the building, it required numerous scan setup and at the same time resulted in considerable data overlap. EU VIGIE2020/654 Study at the request of and financed by the European Commission, Directorate-General of Communications Networks, Content & Technology 13 Success Stories EU Study VIGIE 2020/654 Documentation The onsite scanning at the Cathedral occurred over four visits over 10 months. As each completion stage, a copy of the data was transferred to a workstation for image development and preliminary data registration. The first stage of the process was to develop the associated colour information using the Z+F LaserControl (v. 6.5) software. The software was used to create the individual PNG images and then edited in Adobe Photoshop to address any image colour balance, saturation or artefact issues, if necessary. At the start of the project, it was determined that the data registration would be based on a software-based, cloud-tocloud system as opposed to the extensive use of survey targets. Although the lack of targets could reduce the speed of point cloud registration as well as marginally affect the level of accuracy (especially if a Total Station was used to record the target positions), the cloud-to-cloud method was preferable. It enabled a quicker site setup, avoided the issue of fixing targets onto a large heritage structure, and having to address the problem of people obscuring or remove targets. For production purposes, the data was organised per interior and exterior levels. Trial registrations were carried out using Leica Cyclone (v. 9.1.5) point cloud processing software using automatic and manual alignment. Due to the high number of scans, the registrations were successful with minimal error, as were the bridge/connection points between the interior and exterior datasets. The obvious challenge was to remove all the people and tourists from the grade level scan data as well as general point cloud noise. This was done with the Cyclone software. Data Use In addition, the level of detail given by the dataset ensured precise as-built plans, even concerning the laying pattern of the ashlars. These as-built plans were prepared for a one-week research campaign on-site and will be used for mappings showing information regarding surface deterioration, repairs and replacement of stones, as well as for studies on the architectural structure and deformation. Based on the 3D information of the point cloud and detailed research on the site even exploded assembly drawings can be realised. The research results contributed to a clearer understanding of how this unique medieval portal was erected and modified throughout the centuries. The workflow of using orthographic images of the photorealistic point clouds can be adapted by the Dombauhütte Köln as a basis for site monitoring and for the planning of stone replacements. By using photorealistic and reflectance-based images derived from the point cloud, the CAD results can be optimised. The proposed use for the Cathedral scan and image data is to develop a dimensionally accurate 3D CAD model that would be used in the future planning of the World Heritage precinct. The urban dataset would be used for architectural design development, and more specifically, being able to review the relationship between the new architecture and existing context and the Cathedral. TLS derived data had the advantage of providing the necessary information to develop a 3D urban model, therefore avoiding potentially troublesome intellectual property issues by mixing data owned by other civic organisations. EU VIGIE2020/654 Study at the request of and financed by the European Commission, Directorate-General of Communications Networks, Content & Technology 14 

Success Stories EU Study VIGIE 2020/654 Immovable Success Stories EU VIGIE2020/654 Study at the request of and financed by the European Commission, Directorate-General of Communications Networks, Content & Technology 5