With a mouse click, the virtual flight through the forest begins. However, the oaks in this stand of trees no longer soar skyward: they are extraordinarily robust roof beams. Delicate rays of sunlight shine around them. In this 3D film experience, viewers feel as if they themselves are floating among the oak beams, through centuries of history, through the heart of a country. The “forest” was what the historic beams of the Notre-Dame de Paris were called before they were destroyed by flames on the night of April 15 to 16, 2019. Since that fateful evening only these virtual images can provide an inkling of how special this place once was.
The fire at the Notre-Dame Cathedral, the construction of which dates back to the twelfth century, shook people well beyond Paris. Millions sat aghast in front of their television and computer screens and watched the inferno of flames and the smoke rising above one of Paris’s most beloved landmarks. In the French capital, thousands gathered along the wide safety perimeter set up around the cathedral, which is truly the heart of the city. It is from the square in front of Notre-Dame that all distances to Paris are measured, from all over the world. On that terrible evening, many of the horrified onlookers began spontaneously singing. Some kneeled on the ground, their faces turned towards the burning cathedral.
It wasn’t until the early morning hours that the firefighters managed to fully extinguish the blaze. Then French President Emmanuel Macron stepped in front of television cameras to record a speech that television stations broadcast around the country. “We are a nation of builders,” Macron said in grave tones. “Yes, we will rebuild this cathedral even more beautifully, and I want it to be finished in five years. We can do it.” It was a gigantic promise Macron made on that day. Repairing the heart of an entire country, and in just five years? And if so, how could it be done?
Even before Macron had spoken, one man had already sprung into action on an immensely important part of this task. That very night, Gaël Hamon and his team started searching the archives for existing photos and data from previous surveys of Notre-Dame. Just a short time later, Hamon received a call from the cathedral’s prefect: “We need you,” he was told, “and quickly.”
“The public interest in our work has never been as great as it’s been since the fire at Notre-Dame.”
Shortly thereafter, Hamon and his team were inside the gutted cathedral. Using the techniques of lasergrammetry (laser scanning) and photogrammetry, they made hundreds of color scans of every surface, collecting around 50 billion data points. They mapped inaccessible areas with a drone. Then the photos, point clouds (3D masses of data points) and scans were processed and superimposed on one another. Using these images and current technical data, as well as those from before the fire that were found in the archive, the team used building information modeling (BIM) to prepare a precise model. In this digital process, all relevant information is fed into, combined and recorded in a synchronized database. The result is a three-dimensional model of Notre-Dame, a digital twin with all the dimensions and information about surfaces, materials and their current conditions.
The engineers quickly made the model available to other stakeholders, who have since decided to use it as the foundation for rebuilding Notre-Dame. What’s more, the BIM model is dynamic, continually being updated and adjusted for every structural change on the construction site. Since the beginning, the model has evolved in step with the reconstruction process.
Now, almost three years later, Hamon, aged 51, is sitting in a modern screening room at his company Art Graphique & Patrimoine (AGP) in the suburb of Saint-Denis, north of Paris, and is clicking through the 3D model of the roof truss, which he simply calls the forest. “We were just in the thirteenth century, here we’ve now reached the eighteenth,” he says, commenting on the images. “And you can see it quite clearly: the cut of the trunks is different; you can see steel screws and they no longer have the same shapes.” There’s a photo hanging in this room, in black and white, that looks like an X-ray. It is of Notre-Dame’s spire and timberwork, dating from when the architect Eugène Viollet-le-Duc had the spire rebuilt for the church in the nineteenth century. During the 2019 fire, this spire, which was located above the central nave, collapsed and plunged through the crossing. The blaze also destroyed the lead roof, the tower clock and parts of the ribbed vaults. There was also damage to some of the vault masonry, the roof gable wall between the west towers, the transept façades, two pipe organs, the choir stalls and rose windows from the nineteenth century.
For Hamon, this list of destruction is devastating. He recalls what he felt when he saw the live images of the flames blazing throughout the Gothic cathedral: “I was angry that it was happening. For eight hundred years, we had managed to leave the forest intact. And my generation is responsible burning it down.” Investigations are still underway to determine the root cause of the disaster. What’s clear is that there was a chain of unfortunate events, but probably also some negligence regarding safety precautions. “It took Notre-Dame going up in flames before people finally realized the fragility of our cultural assets,” he says, clearly still upset about it.
He could speak for hours on the topic of restoration and historic site preservation, his great passion. Twenty-seven years ago, the trained stonemason founded AGP as a company specialized in digitizing the documentation of cultural heritage—high-tech for old treasures, so to speak. Today the company employs 33 people—architects, archaeologists, stonemasons, surveyors, art historians, computer graphic artists and software developers. Some of the tools AGP uses are laser scanners (lasergrammetry), which measure surfaces with laser beams, and the process of photogrammetry. The latter uses numerous photos of an object from different perspectives and distances, which are then run through a special software that generates a point cloud of data that is then used to create a digital twin. The company is particularly proud of having been awarded an EPV (Entreprise du Patrimoine Vivant), an “enterprise of living cultural heritage,” by the state – the only company dealing with modern technologies to be recognized in this way. “Our métier is to contribute to the preservation and restoration of historic monuments and artworks, whether it’s a prestigious building such as Notre-Dame or a small chapel in the countryside,” Hamon explains.
Hamon likes to call himself a building radiologist because, like a doctor, he uses imaging techniques to patch things up—buildings, in his case. It isn’t easy. One of the difficulties, he says, is that historic monuments have often been changed or become deformed over time, with some parts now irregular and no longer homogenous, and some even unique—while the modern software solutions and technologies were conceived for modern buildings and can’t even capture certain deformations.
There have been numerous changes in Notre-Dame over the centuries as well, with builders repeatedly reshaping the heart of the nation, a perfectly normal process at the time. Up until the nineteenth century, cathedrals were continually remodeled—evidence of this is Viollet-le-Duc’s rebuilding of the spire, whose collapse during the fire prompted cries of dismay among the horrified onlookers. For special cases such as these, AGP works with beta testers for technology companies such as Autodesk and software manufacturers such as AutoCAD or Revit architecture, whose software is used for BIM or technical drawings.
All the work from the building radiologist Hamon has been added to data from other companies at the now digital construction site of Notre-Dame, which the French National Center for Scientific Research (Centre national de la recherche scientifique—CNRS) set up three months after the blaze. The startup Iconem, for instance, working with Microsoft, launched the “Open Notre-Dame” campaign to collect photos and images of the cathedral, one of the most photographed landmarks in the world. Renowned photographer Yann Arthus-Bertrand provided aerial photos he had taken. The French company Ubisoft did the same, supplying some of the thousands of photos they had taken to create the virtual backdrop for the video game Assassin’s Creed.
Another important data source was the work of American scientist Andrew Tallon, who had used laser scanners to collect billions of data points of the cathedral’s façades and interior back in 2013.
“Never have so many researchers from so many different fields mobilized for a joint project.”
All of this data has now been united under the wing of Livio De Luca. He’s the director of the online platform, which can be accessed by the 175 researchers from a wide variety of disciplines who work at the CNRS, the French Ministry of Culture and also at universities based outside of France. Archaeologists, chemists, engineers, architects and others can all work in real-time. A digital ecosystem has been created that is unique the world over.
With the help of this platform, the actual physical construction site and the digital site are proceeding in parallel. A BIM model enables the ongoing sharing of all information, the modeling of different options and the validation of decisions. It’s even possible to determine the exact placement for a crane or scaffolding without having to be onsite. This saves a lot of time, with the next phase of reconstruction set to begin soon.
Even when the heart has finally been rebuilt, the work of all the engineers and researchers will be preserved. Starting in 2024, it is planned to open the database to the whole world. Any company that wants to make a film or a video game from the information will be allowed to use the material. So in the end, perhaps there may also be a silver lining to the fire: Notre-Dame will soon exist in two copies, once again in analog and, thanks to clever minds like Hamon’s, for the first time in a completely digital form.
