How Notre Dame is being rebuilt from 50 billion scraps of data

How Notre Dame is being rebuilt from 50 billion scraps of data

The day after the catastrophic Notre Dame fire, the French government turned to Art Graphique et Patrimoine for help

Art Graphique et Patrimoine

On April 15 2019, Notre Dame de Paris, the monumental cathedral in the heart of the French capital, lost its roof to a fire. Flames destroyed Notre Dame’s entire “forest” – the roof’s 300-year-old wooden structure – and its 19th century spire.

A digital memory of the “forest”, however, lives on. On YouTube, a video walks you through a 3D model of the century-old frame, where clerics walked long before Victor Hugo wrote about lovelorn bell-ringer Quasimodo. That’s thanks to Art Graphique Patrimoine (AGP), a French company specialising in 3D digitisation and modelling of cultural heritage monuments, which had scanned the entire structure between 2014 and 2016. The day after the fire, the French government asked the company to draw on its archives to build a digital version of Notre Dame as it had been before parts turned to ashes.

“Our data covered most of the building, including all of the exterior, the two bell towers, the organ, and the entirety of the ‘forest’,” says Gaël Hamon, a stonemason and the CEO of AGP.

To scan a heritage site, AGP uses helicopters, drones, and terrestrial scanners. The team’s specialists in lasergrammetry and photogrammetry (techniques allowing for precise measurements using 3D rangefinders and photographs) then turn the measurements into scatter plot models of the building. Digitally recreating the cathedral from the various scans in their archives took AGP two months, six ‘super calculators’ (computers that turn data points into scatter plot models), and the 21-strong team’s full-time attention.

AGP’s data set of Notre Dame is composed of 50 billion 3D points, or one to two points by square millimetre. The model for the “forest” alone counts 3 to 5 billion points, captured in 150 different scans of the structure. But that’s just the start: now that the pre-fire cathedral lives on digitally, AGP is working on an extremely detailed 3D “smart database” of the building as it is now, with integrated information for each architectural element. Zooming in on a stained-glass window, for instance, would list its materials, date of construction, style and any other relevant information. The technique, called Building Information Modelling (BIM), will map a 3D model of the damage and allow for simulations of reconstruction works. AGP’s BIM modelling of the cathedral is due to be completed in early 2020, and no restoration work is expected to start before then.

“We are treating Notre Dame the way a greatly traumatised patient would be treated in hospital,” Hamon says. “With the BIM, it’s getting everything: MRI and medical scans, blood tests, a full check-up. We can’t heal it without a proper diagnostic.” Not only will the modelling ensure that the ‘old’ Notre Dame lives on digitally: it will be the essential blueprint for the cathedral’s restoration.

The Notre Dame fire was a tragedy, but it did help advocate for the pioneering techniques Hamon has been working on for 25 years. “A few years ago, our work interested no one,” he says. The company almost went under several times. “Yet, today, it’s great luck we have worked on the cathedral so much over the years.” Today, the company has worked with the management of many heritage sites – for technical measures like Notre Dame’s, but also for cultural projects, such as the Roman amphitheatre in Orange, Provence, which AGP recreated in VR for tourist visits. Back in 1994, when Hamon founded AGP, scanning monuments involved taking analogue photos of the buildings. “We had to enter all the data, all the measures, by hand,” Hamon recalls. “Now we have digital photos, we put everything in the algorithm – it’s automatic.”

Since then, AGP has worked on over 2,000 monuments in 18 countries, including the Royal Albert Hall, the Palmyra ruins in Syria, and the Louvre museum in Paris. Their archives have helped restoration works: recently, when the rose window of the Soissons cathedral exploded in a storm, AGP’s model was used as a reference.

Soon the company hopes to open a cultural space offering VR visits of heritage monuments. But right now, it’s all about Notre Dame. “Our measures will give architects the necessary tools to make a ‘before/after’ diagnostic for the restoration,” Hamon says. Healing a cathedral that took over a century to build cannot be rushed.