What is... 3D laser scanning?
The world watched as Notre Dame Cathedral’s roof burned. Another kind of laser focus could help bring it back to life
The scans that could help rebuild Notre Dame.
Andrew Tallon described himself as a geek – a “tacklehead” was his word of choice when National Geographic came knocking in 2015. The art historian paired his tech skills with his love of medieval architecture in an epic project that would prove prescient: scanning with lasers every point of Notre Dame Cathedral.
Years later, after Tallon passed away from cancer, the Parisian monument caught fire. In the end, only the roof was lost, but at points it appeared as though the entire building would burn, taking with it centuries-old stained glass and other treasures. Here’s how his lasers work – and how they could help the rebuilding efforts.
What is laser scanning?
This 3D scanning is essentially the same as the lidar (light detection and ranging) systems used in driverless cars. A laser beam is pulsed and steered with prisms, deflectors or mirrors. As the beam bounces off something, be it a flying buttress or a church pew, the distance is recorded. Repeating that millions and millions of times and reassembling the data digitally lets any object be drawn in minute detail.
Why did Tallon scan the cathedral?
Notre Dame isn’t the first epic scanning project. Lasers have been used to capture near-perfect records of the Science Museum’s Shipping Gallery, before it was packed away, and to piece together digital versions of ruins destroyed by war in Syria. Moreover, they have revealed the city beneath Rome, with 3D scans of archaeological findings layered onto the existing landscape.
However, Tallon wasn’t looking to preservation but to deconstruction – digitally, at least. The project sought to track how the building shifted and moved in order to understand better how it was constructed. The data points collected by the lasers were precise enough to capture the entire cathedral in 5mm increments.
How did the process work?
The laser scanner is mounted on a tripod, with the beam measuring distance and other data precisely. Tallon’s scanner of choice was from Leica Geosystems. Over five days of work, Tallon collected more than a billion points of data from 50 different positions. Normally, that data is assembled into a point cloud, creating a ghostly image of the structure. Tallon went a step further and took panoramic photos from each tripod position, mapping the laser data to pixels in the images in order to colour the final image.
Is it enough to rebuild the roof?
The laser images created by Tallon are detailed enough to see exactly how the roof was built, including the flaws that made it less stable than it could be. That means that his scans could help improve the new roof, although it’s not yet clear if the rebuilt version will exactly mimic the lost version or have a fresh design. If more of the cathedral has burned, his scans could also have helped the restoration efforts inside.
Is this the only scan of Notre Dame?
Remarkably, it’s not. An artist working on a video game called Assassin’s
Creed Unity rebuilt the cathedral in painstaking detail with the help of historians and historical photos. Thanks to 3D scanning, even if something burns, it lives on in the digital world.