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Notre Dame – Historian uses lasers to unlock mysteries of Gothic cathedrals

Thirteen million people visit the Notre Dame Cathedral in Paris every year, entering through massive wooden doors at the base of towers as solidly planted as mountains. They stand in front of walls filigreed with stained glass and gaze at a ceiling supported by delicate ribs of stone.

If its beauty and magnificence is instantly apparent, so much about Notre Dame is not. To begin with, we don’t know who built this cathedral—or how.

The bishop of Paris, Maurice de Sully, commissioned the massive church complex around 1160. Yet the names of those who first constructed this masterpiece are lost to history. They left no records—only centuries of speculation—behind.

A former composer, would-be monk, and self-described gearhead—or, as he puts it, “tacklehead”—Tallon intends to make that history right. With the help of 21st-century laser scanners, he is teasing out clues hidden in the ancient stones of Notre Dame and other medieval structures—and revolutionizing our understanding of how these spectacular buildings were made.

[Andrew] Tallon, who died Nov. 18, 2018, at 49, wasn’t the first to realize that laser scanners could be used to deconstruct Gothic architecture. But he was the first to use the scans to get inside medieval builders’ heads.

”If I had texts at every point, I could look in the texts and try to get back into the heads of the builders. I don’t have it, so it’s detective work for me.” – Andrew Tallon, art historian.

“Every building moves,” he says. “It heaves itself out of shape when foundations move, when the sun heats up on one side.” How the building moves reveals its original design and the choices that the master builder had to make when construction didn’t go as planned. Tracking this thought process requires precise measurements.

For a long time, the tools used to measure medieval buildings were nearly as old as the buildings themselves: plumb bobs, string, rulers, and pencils. Using them was tedious, time- consuming, and error-prone.

“You can’t hang from a vault and measure it by hand,” says Michael Davis, an art historian at Mount Holyoke College who once spent nine weeks surveying two churches with these primitive tools.

Laser scans, with their exquisite precision, don’t miss a thing. Mounted on a tripod, the laser beam sweeps around the choir of a cathedral, for example, and measures the distance between the scanner and every point it hits. Each measurement is represented by a colored dot, which cumulatively create a three-dimensional image of the cathedral. “If you’ve done your job properly,” says Tallon, the scan is “accurate to within five millimeters [.5 centimeter].”

Two researchers at Columbia University—Peter Allen, a computer scientist, and Stephen Murray, an art historian—attempted one of the first laser scans of a Gothic building in 2001 at the cathedral in Beauvais, north of Paris, which Murray had once measured by hand with steel tape and wooden calipers.

Unfortunately, the scanner “actually went up in a puff of smoke. It really did emit smoke,” says Murray. “And at that point people didn’t know how to render [the data] into a three-dimensional model that was manageable.”


Tallon figured out how to knit the laser scans together to make them manageable and beautiful. Each time he makes a scan, he also takes a spherical panoramic photograph from the same spot that captures the same three-dimensional space. He maps that photograph onto the laser-generated dots of the scan; each dot becomes the color of the pixel in that location in the photograph.

As a result, the stunningly realistic panoramic photographs are amazingly accurate. At Notre Dame, he took scans from more than 50 locations in and around the cathedral—collecting more than one billion points of data.

Tallon, says Murray, his Ph.D. adviser at Columbia, is “able to combine that astonishing grasp of technology with the big humanistic vision that one hopes that art historians have.”

The laser scans have led to surprising new information about Notre Dame’s builders. For one thing, they sometimes took shortcuts. Even though medieval builders strove to create perfect dwelling places for the spirit of God, Tallon’s scans reveal that the western end of the cathedral is “a total mess … a train wreck.” The interior columns don’t line up and neither do some of the aisles. Rather than removing the remains of existing structures from the site, the workers appear to have built around them.

That cost cutting could have been catastrophic. Based on stylistic changes, scholars have long suspected that work on the western facade stopped for a while before the towers could be built. When Tallon scanned it, he discovered why. The Gallery of Kings—the line of statues above the three massive doorways—was almost a foot (.3 meters) out of plumb. Tallon concluded that the western facade, built on unstable soil, began leaning forward and to the north. Construction had to be halted until the builders could be confident that the ground had compressed enough to resume. After an anxious decade or so, it had.

The builders were more sensible when it came to constructing flying buttresses, which some scholars have argued were added after the cathedral was built. After measuring the walls, Tallon determined that the flyers, as he calls them, were part of the cathedral’s original design. The vaults in the ceiling should push walls outward but “the upper part of the building has not moved one smidgen in 800 years.” The reason? “The flying buttresses were there from the get-go,” pushing the walls inward and creating a stable balance of forces.

Though most of the structure is perfectly plumb, the great columns at the center of the cathedral were built ever-so-slightly outwards, and the choir doesn’t align exactly with the nave. To Goodyear, imperfection “was the secret sauce,” says Tallon, “that medieval folks sprinkled on their buildings to make them beautiful.”

Tallon believes the true “secret sauce” was faith. “There was a biblical, a moral imperative to build a perfect building,” he says, “because the stones of the building were directly identified with the stones of the Church”—the people who make up the body of the church.

“I like to think that this laser scanning work and even some of the conventional scholarship I do is informed by that important world of spirituality,” says Tallon. “It’s such a beautiful idea.”


Andrew Tallon has used lasers to scan more than 45 historic buildings, including this cathedral in Chartres, France – PHOTOGRAPH BY ANDREW TALLON


Fortunately, there are incredible 3D scans of Notre Dame. The good news: we have a highly-detailed digital template for how to rebuild.

For his scans of Notre Dame, Tallon recorded data from more than 50 locations in and around the cathedral, resulting in a staggering one billion points of data.

Each scan begins by mounting the laser onto a tripod and placing in the center of the structure. The laser sweeps around the area in every direction, and as it hits a surface, the beam bounces back, recording the exact placement and surface of whatever buttress or column it landed on by measuring the time it took the beam to return.

Every measurement is recorded as a colored dot, combining together into a detailed picture, like the color pixels of a digital photograph.

Eventually those millions of dots form a three-dimensional snapshot of the cathedral, and the resulting images are meticulously precise; if the scan is done properly, Tallon told National  Geographic, it should be accurate within 5 millimeters.