A 2012 scan of the western frontispiece of the Cathedral of Notre Dame Andrew Tallon

When the art and architectural historian Andrew Tallon embarked on a laser scan of the Cathedral of Notre Dame in 2010, he little imagined that a devastating fire would one day throw his work into relief. Five months after the blaze that immolated the cathedrals roof and spire, a crowd of around 150 gathered on a recent afternoon at the Frick Collection in New York to hear more about the scan, which could prove useful in mapping Notre Dames restoration.

Lindsay Cook, a visiting assistant professor of art at Vassar College for whom the Belgian-born Tallon was once a mentor, paid tribute to his pioneering work in an hourlong lecture. (Tallon died of brain cancer in November 2018 at age 49.) She recounted how in 2010, with the financial backing of the producers of a European arts documentary, Tallon set out to document Notre Dame from top to bottom with a laser scanner manufactured by Leica Geosystems. In 2012, he returned to the cathedral with a more highly refined scanner to capture the details of the churchs western frontispiece.

A laser scanner in the process of mapping the western frontispiece of the Cathedral of Notre Dame in 2012 Andrew Tallon

The scanning process, Cook explained, begins by placing a series of reflective markers on the surface of what it is capturing, be it a series of ribbed vaults or carved figures. Then the scanner sends out a laser beam to each marker, measuring the distance between every point the laser hits and the scanning device itself. A mass of “point cloud” data results, creating a three-dimensional record of the structure with a margin of error of only five millimeters.

“The primary reason he wanted a laser scan of Notre Dame of Paris was to understand more fully the nature of Gothic structure and to pinpoint structural anomalies in the cathedral,” Cook said. “He was as interested in irregularity as he was in regularity.”

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Some of those irregularities are visible to the naked eye, she said. She cited the piers on the south side of the cathedrals nave, which do not seem to line up with the piers of the north side. The laser scanner simply mapped this irregularity more precisely than it had ever been before.

Nonetheless, the scan served as a corrective. “For generations, plans of Notre Dame were tidied up, or corrected to appear more regular than the building actually is,” she said. “In reality, the cathedrals ground plan is far more idiosyncratic.”

Yet the scanner also captured an astonishing example of regularity. One of Tallons most striking discoveries involved the cathedrals single-span flying buttresses, which some scholars have theorised were added years after the cathedral was built in response to structural problems. If that were correct, Cook said, one would expect the upper walls of the choir to have splayed out because of a lack of stability before the buttresses were added.

But Tallons scan proves otherwise, definitively settling the matter. “The 2010 laser scan showed that the walls were more or less perfectly straight, or in plumb, and thus must have been supported by flying buttresses from the outset,” she said. The result vindicates the Columbia University art historian Stephen Murray, who had hypothesised that the buttresses had been envisaged from the start of construction around 1160, Cook said.

A book dissecting some of the laser results was published in 2013: Notre-Dame de Paris: Neuf Siècles de l'Histoire, authored by Tallon with the researcher Dany Sandron, which also proposed hypothetical states of the building as it evolved under construction in the 12th and 13th centuries. Next spring, Cook's English translation of that book, tentatively titled Notre-Dame of Paris Through Time is to be published by Penn State University Press.

Significant concern has arisen since the fire that Notre Dames upper vaults were destabilised by the searing heat from the blaze followed by the torrents of cold water applied by firefighters. Going forward, Cook said, the scans could be used by the cathedrals restorers as a “snapshot” of its structure in 2010-12 and point to any subsequent movement as a result of the fire. In early May, she noted, a company conducted a sweeping new laser scan of Notre Dame that could be compared with Tallons scan results.

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