Twenty-First Century Gothic

Thanks to 3-D scans of Notre Dame made before and after this year’s catastrophic fire, we have an incredible wealth of information to aid in the restoration of an architectural icon.

An axonometric view of two bays of the Notre Dame choir from the 2010 laser scan

Photo: Andrew Tallon

On the evening香蕉视频苹果下载 of April 15, 2019, a fire alarm sounded during Mass at Notre Dame, the cathedral of Paris, prompting its evacuation. This was a prelude to a cultural catastrophe. By the next morning a damage assessment could begin in earnest. Gone was the 13th-century wooden roof, its beams composed largely of squared-off oak trees; Eugène Emmanuel Viollet-le-Duc’s 19th-century spire, which had broken through the central crossing vault; the majority of that vault and some segments of the vault webbing of the transept arms; and the lead covering of both spire and roof, which had melted and poured into the interior through the gaping holes in the vaults.

Talk quickly turned to restoration, with many arguing for a contemporary design to replace the roof and spire. A bill regulating the conservation and restoration passed both Houses of Parliament and was signed into law in late July by President Emmanuel Macron. Contrary to the wishes of some senators, the law does not explicitly state that the restored cathedral must look as it did before the fire, but it does order the conservation and restoration to preserve “the historic, artistic and architectural interest of the monument.” Moreover, the architect in charge of the project, Philippe Villeneuve, has indicated that he will restore the spire à l’identique—identically.

But how? By happy accident, there exist 3-D laser scans of the entire cathedral. They were made by Vassar College architectural historian Andrew Tallon in 2010 and 2012 for two television documentaries. (Tragically, Mr. Tallon died last year.)

Laser scanning is a means of measuring an entire edifice, providing a digital record with a margin of error of only five millimeters. First, a series of reflective targets is placed on the surface of a building. Then, the scanner is positioned in dozens of different spots to capture every nook and cranny. In PBS’s “NOVA” special “Building the Great Cathedrals,” Mr. Tallon explained what came next: “[The scanner] sends a little laser beam out from its eye, and it measures a thousand times a second the distance between itself and whatever it’s hitting, and so, as it slowly pans across the wall, it’s shooting this laser out and taking a whole series of measurements, which are then represented in three dimensions as a series of X-Y-Z coordinates.” The result is a mass of data known as a point cloud. That data set may then be viewed using analytical software and “sliced” into smaller pieces for closer inspection.

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