The major fire at Notre Dame Cathedral in Paris on April 15, 2019 affected people all over the world. Today, half a year after the fire, bricks continue to fall from the arches of the church, which is still at risk of collapsing.
But there is hope. Notre Dame was documented from inside and outside in millimeter-precise laser scans. Architecture historian Andrew Tallon, who died in 2018, scanned the cathedral in 2010 from 50 different perspectives, using a Leica ScanStation C10. His objective was to investigate in the scans the architectural structures, structural engineering calculations, and the exact procedures used by the architects. In the five-day field campaign, more than a billion data points were aggregated: The basis for a precise 3D model of the intact church. Tallon linked the laser measurements with reference measuring points and photographs of the scanned areas. His scans produced a realistic, color-accurate basis for the reconstruction of the burned down cathedral. In addition to Notre Dame, the scientist also created digital 3D models of various Gothic buildings – always looking for the secrets of their building history, construction procedures, and any deformation and settlement movement that the architects had to deal with during the decades-long construction phases.
3D laser scanners were also used in the days following the devastating fire. Experts from French company Art Graphique et Patrimoine (AGP), who had cooperated with Andrew Tallon in the past, arrived with FARO Focus laser scanners to measure Notre Dame. The team will compare the data with the reference data to investigate the total extent of the damage and create a building information model (BIM) for the cathedral. In building and infrastructure projects, the construction industry increasingly works with models to simulate the construction work. BIMs help minimize risks and errors and optimize construction planning. BIMs are also a binding planning basis for all the different trades that are involved: a digital twin in which every constructional change is documented.
3D laser scans especially provide the basis for BIMs of existing buildings. In the case of the burned down cathedral in Paris, contactless laser surveys have another crucial advantage. The experts were able to carry out the measurements within one day without the risk of having to enter the building, which still could collapse. During this immediate action, AGP prepared more than 300 color scans with almost 40 billion data points. The team also used a drone to deliver photogrammetric image data of the cathedral. AGP and FARO can now benefit from the fact that during the project with Tallon, they also carried out numerous measurement series in Notre Dame. As a result of this, they have extremely accurate models of the areas of the church that suffered most. For example, they claim to have scans of the destroyed wooden beams, which alone contain up to 5 billion data points – in many cases several dots per square millimeter.
Experts still complain that the BIM modeling tools are not sufficiently accurate for the organic deformations of sacred buildings. This probably explains why systematic documentation of historic buildings with laser scanners is only just beginning. However, there are various approaches. For instance, in 2016, the Cologne Cathedral Workshop, together with Fresenius University of Applied Sciences in Cologne, Heriot-Watt University, Edinburgh, Scotland and laser scanner manufacturer Zoller+Fröhlich created a digital 3D model of Cologne Cathedral. Over a period of 225 working hours, during which sport climbers also transported 3D scanners onto the cathedral towers to carry out measurements there, a total of 635 scans were produced with two terabytes of data. The equipment used was a Z+F IMAGER® 5010X, which measures more than one million pixels per second and achieves measuring accuracy in the sub-millimeter range at distances of up to 187 meters. The laser scanner uses sensors to determine its exact position and sends this, together with the scanning data, to software which automatically creates a 3D image of the scanned object. During scanning, the data is also checked for completeness so that any gaps can be closed immediately.
The Cologne Cathedral Workshop will use the millimeter-precise complete documentation of the inside and outside of the building as a basis for restoration work and it will also help the experts to assess and repair damage caused by weather and age. It also means that a complete reference data record is now available for the cathedral. By using this for comparison, the experts from the Cologne Cathedral Workshop can see exactly how the structure changes over time. One thing is certain: We don't need a catastrophe to appreciate the significant advantages of laser scans.