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07/12/2019

Precision Laser Scans of Notre Dame Cathedral Can Help to Preserve, Restore, and Rebuild

Notre Dame Cathedral

On Monday, media across the globe focused on the Notre-Dame cathedral fire in Paris. An iconic piece of Gothic architecture and a source of national pride and identity for France, the losses sustained by the fire are incomprehensible.  

While the famous bell towers were saved and many important artworks and artifacts were evacuated from the structure, flames destroyed the roof and spire of the cathedral and ensured that any restoration and rebuilding process will be a difficult and complex challenge. The extent of the damage, while obviously significant, has yet to be determined. 

However, despite the tragedy, there is some hope for a faithful restoration of the cathedral. In 2010, Andrew Tallon, an architectural historian and professor at Vassar College, captured the entire structure with a Leica Geosystems laser scanner.  

Unfortunately, Tallon passed away last year, but the work he left behind is an important set of data that can help restoration experts determine how to rebuild the cathedral.  

The orange circles on this map represent the scan locations. Click on them to view the perspective of the scanner. From there, you can select the ruler icon and take measurements within the cathedral.

Notre-Dame Captured in One Billion Points of Data 

Andrew TallonWith a tragic cultural loss like this, there simply is no replacement for the original work in the cathedral — especially wooden and glass features and structural components. Yet laser scanning helped Tallon to create an accurate 3D model of the Notre-Dame based on point clouds, and he captured everything, from flying buttresses and rib vaults to stained glass windows and intricate wood carvings. Journalist Alexis C. Madrigal further explains the process in The Atlantic

“They mounted the Leica [Geosystems] on a tripod, put up markers throughout the space, and set the machine to work. Over five days, they positioned the scanner again and again—50 times in all—to create an unmatched record of the reality of one of the world’s most awe-inspiring buildings, represented as a series of points in space. Tallon also took high-resolution panoramic photos to map onto the three-dimensional forms that the laser scanner could create.”  

 The key word above is reality: Tallon captured the cathedral as it was on the days that he scanned it. Reality capture of the Notre-Dame will reveal ways that the structure has changed over time due to internal and external forces, down to one-millimeter accuracy. And the data obtained by laser scanners offers far more precise measurements than any drawing, modern or historical, ever could: 

For a structure such as Notre-Dame, built over hundreds of years, it’s almost certainly the case that any drawing or archival material about its construction would be incomplete or incorrect, the preservationist and architectural historian Lindsay Peterson told [Madrigal]. Given that, the laser data might be the ground truth in a way that nothing else is.

While any restoration efforts simply cannot replace the handcrafted work completed so many centuries ago, Tallon’s scans can help bring today’s rebuilders as close to the originals as possible. And the “record of reality” from Tallon’s work can help restoration professionals determine the best course of action for stabilizing and rebuilding the Notre Dame. 

Notre-Dame Restoration Efforts: New Data Revealed in Tallon’s Laser Scans

To begin restoration efforts, professionals will not only need to understand the cathedral as it existed prior to damage, but also how the structure changed over time. Tallon’s scans revealed much more about the cathedral than previously known. Marie-Caroline Rondeau further explains the complex data obtained by laser scanning in The Global Magazine of Leica Geosystems: 

"Gothic buildings, like most buildings, were constructed along the plumb-line. The parts of the building that are no longer plumb were thrust out of line by the combined forces of the vaults, arches and wind – direct evidence of invisible forces working on the walls. By quantifying these movements using 3D laser scanning, with a precision of several millimeters, it has become possible to speak with confidence about what has actually happened in the building, and what decisions were made by the builders during construction to tame these unwanted deformations."

In fact, discovering an unrecorded history of how the structure changed over time was one of Tallon’s chief goals. Writer Joel Hruska explains how useful this information is on ExtremeTech

"At the time, his goal was to uncover clues about the building’s construction and renovation process that weren’t preserved to the present day. His work in 2014-2015 uncovered that the Gallery of Kings had shifted almost a foot out of plumb and that this area of the cathedral might have been left untouched for as much of a decade before work began again, giving the soil time to settle. His work also showed that the internal columns of Notre Dame aren’t in perfect alignment and that the church may have incorporated existing structures in its design rather than knocking them down first."

Laser Scanning Technology and Historic Preservation

Scanning Notre Dame CathedralSuch detailed information will likely be of immense importance to restoration efforts, especially when considering structural components of the cathedral and how to stabilize them. We can also anticipate that laser scanning may be used in the coming days and weeks to capture the damage in the cathedral—which then could be compared to Tallon’s scans. 3D point cloud models of the cathedral from before and after the fire may be the best resources available for professionals tasked with one of the most complicated rebuilding projects of our time. 

While the restoration work on the Notre-Dame is already estimated to take 10-15 years, having point cloud data derived from laser scans at least gives restoration experts a strong starting point. Because of the precision and scale of laser scanning technology, Tallon’s work likely forms the most accurate and comprehensive measurement and modeling of the Notre Dame to date. 

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