A billion laser points helped bring Notre Dame back to life

CNN
—

After a catastrophic fire five years ago, Notre-Dame de Paris Cathedral reopened this month and looks much the same as it did when it was first built in 1163.

The massive reconstruction project was a testament not only to the hard work of the French, but also to the lasers, drones and other advanced technologies that gave rebuilders a glimpse into the building’s past.

“The time frame would not have been possible without recording what existed,” Amy Bunszel, executive vice president of architecture, engineering and construction at 3D software company Autodesk, told CNN. Your company was instrumental in creating a model of the building as it existed before the fire and gave the reconstruction work a kind of guide for further measures. “It would have required a lot more guesswork. Imagine taking millions of tourist photos (for reference) instead of just having one consolidated, perfect representation.”

Technology helped France meet President Emmanuel Macron’s ambitious five-year goal for rebuilding the landmark. This required teams from multiple companies that combined damage assessment rounds, custom-built maps, and advanced techniques now used in everything from film animation to construction.

And it also took a bit of luck.

In some ways, the rebuilding began before the fire. Fortunately, in 2015, art historian Andrew Tallon carefully scanned the building with lasers. Tallon, who studied Gothic architecture, sought to understand how medieval builders built some of Europe’s greatest cathedrals.

His first attempt – four years before the cathedral was engulfed in flames – required 12 lasers and a team of seven engineers to scan the building and collect 46,000 images, according to Bunszel. The spatial map he created used more than a billion laser-measured points and revealed some previously unknown details about the cathedral – such as that the interior columns at the western end of the cathedral are out of alignment.

Tallon died in 2018, just months before the April 2019 fire that shocked Paris. As hundreds of firefighters extinguished the flames, most of the structure was destroyed, including the iconic 315-foot tower, which collapsed through the roof.

While his detailed scans revealed much about the ancient structure – which has undergone countless modifications and minor changes over the centuries – they alone were not enough to create the detailed model needed to restore Notre Dame.

This is where Autodesk came into play. After the fire, Autodesk worked with French laser company AGP to install scanners around the cathedral and capture billions of points needed to create a full-scale digital model. The companies donated their services to Rebâtir Notre Dame, the public body that led the restoration effort.

The post-fire process didn’t always go smoothly, Bunszel said.

“The cathedral was very unstable right after the fire,” Bunszel said. “They had to build temporary structures and continuously scan during the rebuilding process.”

Ultimately, the full view was captured by overlaying new laser scans and drone footage onto Tallon’s previous scans.

Due to Notre Dame’s complexity, structural details and size, it took the companies over a year to create the newer 3D model. Compared to traditional methods for documenting historical buildings, such scans speed up the process significantly. Notre Dame also had to be continuously scanned.

Although the new cathedral looks nearly identical to the original, some areas have been modernized: the addition of sprinkler and fire suppression systems, optimized lighting placement, and a cleaner appearance thanks to years of reduced soot buildup from the use and operation of candles.

The square in front of the cathedral was also redesigned using Autodesk technology. The company helped four competing teams create 3D visualizations of their proposals before receiving public input, and ultimately a winner was selected.

Founded in 1982, Autodesk has a long history of using 2D and 3D modeling for everything from film animation and video games to construction. Its scanning technology has played a key role in disaster recovery and historic preservation in particular. After a massive earthquake damaged the cathedral in Christchurch, New Zealand, in 2011, the company’s technology helped rebuild the structure using drones and a robotic dog — equipped with a mounted laser — that scanned unsafe areas.

In the US, the same technology helped create a digital replica of the Michigan State Capitol and historic downtown Brownsville, Pennsylvania, to support preservation efforts.

Wladek Fuchs, a professor of architecture and community development at the University of Detroit Mercy, believes more governments and institutions should consider scanning buildings in the event of a tragedy or natural disaster. He is currently involved in a project to reconstruct the ancient city of Volterra, Italy using 3D modeling in collaboration with the Volterra-Detroit Foundation’s Digital Preservation Workshop.

“Disasters happen in unexpected places, but if the most treasured structures of our cultural heritage can be documented in this way, the impact will be less devastating,” he said.

For Notre Dame Cathedral, the latest 3D model is not only a tribute to the past, but also an investment that will pay off for years to come.

“In the future, they will have a 3D representation of the building that they can use for maintenance,” Bunszel said, “and can also plan for the future.”

Michelle Lou and Brandon Griggs contributed reporting.