Insights and long-term value of the Notre-Dame Cathedral 3D project

May 29th, 2019, Published in Articles: PositionIT

In 2011, the late Prof. Andrew Tallon used 3D laser scanning to map the Notre-Dame Cathedral. On 15 April 2019 the conflagration of the cathedral’s spire highlight the cultural and historical value of a geospatial technologies such as 3D laser scanning, far beyond its initial value of understanding the construction of Gothic architecture.

The advent of Gothic art in the twelfth century marked the start of an extraordinary architectural adventure in the Middle Ages. Cities in France – and then throughout Europe – vied to build the most perfect image of heaven on earth. The master builders of Gothic architecture conquered hitherto unknown heights with feats of technical bravado, as astonishing today as they must have been then, but left no hints, other than those hidden in the buildings themselves, as to what their process of design might have been. Traditional means of understanding Gothic structural design using modelling are fraught with conceptual and technical challenges.

Andrew Tallon, Professor at Vassar College (New York) and specialist in Gothic architecture, adopted 3D laser scanning technology to circumvent these problems – to read the stories of the builders directly in the stone and mortar of the buildings, with millimetre precision.

Fig. 1: A bird’s eye view of the Chartres laser scan (not unified).

Fig. 1: A bird’s eye view of the Chartres laser scan (not unified).

Structural models have been used since the 1960s to explore the questions that art historians have long asked about Gothic buildings. However, the lack of precise methods of measurement, the assumptions necessary to create a workable model and the lack of documentary evidence have rendered conclusions too divergent. The invisible forces affecting the walls have inspired multiple theories based as much on intuition as on material evidence. It seemed time to tell the story of Gothic buildings anew.

Revealing invisible forces

In trying to achieve this goal, Prof. Tallon sought funding from the Samuel Kress Foundation to undertake a large-scale laser survey at the cathedral of Bourges in 2008. By creating a highly accurate spatial map of the building it became possible to examine, in the finest detail, every unintended departure from rectilinearity.

Fig. 2: Cross section of the Chartres in France.

Fig. 2: Cross section of the Chartres in France.

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 millimetres, 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. Evidence from the cathedral of Bourges, for example, reveals that its builders actively struggled to keep the piers in perfect alignment by adjusting the position of subsequent layers of masonry; they eventually resorted to installing iron ties, hidden above the vaults, to arrest the tendency of the vaults to push the building out of plumb.

Evolving technology

Tallon’s interest in 3D laser scanning goes back to 2001 when his dissertation director, Columbia University Professor of Art History, Stephen Murray, teamed up with Peter Allen, Professor of Computer Science at Columbia, to produce the first laser scan of Beauvais Cathedral using one of the first Cyrax laser scanners.

Fig. 3: Laser scanner equipment outside Notre-Dame Cathedral in 2010.

Nearly ten years later, Tallon, assisted by Paul Blaer, a colleague of Allen’s, used one of the first Leica ScanStation C10s available in France to produce a high-density scan of the cathedral of Notre-Dame in Paris in the context of a documentary that aired in Europe on the television station Arte in 2011. After having seen the data that other 3D laser scanners produce, Tallon was convinced of the superiority of the C10 in terms of long range and accuracy.

The question of speed was also of critical importance. The cathedral in Paris is visited by approximately 13-million visitors per year (an average of more than 30 000 people per day): time spent scanning the entire monument (with an area of 4800 m2 and a total length of 128 m) had to be reduced to a minimum so as not to disturb both the daily liturgy and the flocks of tourists.

Thanks to advances in technology then, such as the onboard interface of the Leica ScanStation C10 and new functions in the Leica Cyclone software – particularly the advances in cloud-to-cloud registration – Tallon was able to be far more productive during his missions. He explained that he was able to produce twice as many stations including target referencing with the Leica ScanStation C10 as he had with a Leica HDS3000. For example, he and his team were able to complete the scanning work – over 50 stations and more than 1-billion textured data points – at Notre-Dame in only five days. At the time, he also looked forward to being able to work with the newest Leica Geosystems laser scanner then, the P20, given its improvements in acquisition speed.

“This type of work simply would not have been possible before the 3D laser scanner,” Tallon said. “Manual measurement would have required extensive scaffolding and months of work to accomplish – not to mention the inevitable errors due to imprecision.”

Fig. 4: The question of speed was also of critical importance given the great number of visitors the cathedral receives.

Tallon emphasised that the importance of laser scanning lies well beyond its ability to simply measure. “The consequences of laser scanning are equally far-reaching in terms of representation. A 3D laser-generated model allows one not only to peer into the building but also to displace it, measure it and, most importantly, immerse oneself in its spatial and structural matrix. Both the structural skeleton and the evidence of its malfunction, the deformation produced by the push and pull of its constituent elements, are made immediately apparent to expert and lay audiences alike – a picture, in this case, well worth a thousand words.”

Fig. 5: The view from Quai de Montebello with the Notre-Dame Cathedral spire aflame. (Credit: Wandrille de Préville)

On 15 April 2019 the Notre-Dame Cathedral’s spire caught fire, the cause of which is still being investigated. The fire caused structural damage to the cathedral, although fortunately important elements of the structure and most of the church’s treasures such as artworks and more were saved.

Legacy of a laser scan pioneer

Even as firefighters sifted through the ashes at Notre Dame Cathedral in Paris, seeking clues about the cause of the tragic blaze, Catholic clergy and French officials were turning their attention to the massive rebuilding project. Many experts in the field noted the key role the work of a renowned architectural historian, the late Vassar associate professor of Art Andrew Tallon, may play in the restoration.

Tallon, who died of cancer on 18 November 2018, pioneered the use of laser technology and advanced imaging techniques and built a digital model of Notre Dame. His work with images captured by the drone-borne, 360° spherical cameras he deployed at the 800-year-old cathedral continued until his death. In a publication following his death, the Society of Architectural Historians said Tallon “was esteemed worldwide as an innovative scholar of French Gothic art and architecture, one who introduced new digital techniques to the analysis and re-creation of the spatial archaeology of medieval buildings. In all his work, he was an inspired and generous educator who brought the past to life in vivid and meaningful ways.”

As news of the tragic fire spread throughout the world, engineers and architects alluded to Tallon’s unique, detailed analysis of Notre Dame as they discussed the daunting rebuilding project. This data could now be invaluable in the reconstruction.

Tallon’s book on the cathedral, co-written with Dany Sandron, tells the history of the construction of the building in visual terms. The primary illustrations were created from the laser scans acquired in 2010 and 2012. For more information please visit www.gothicstructure.org.

Aknowledgement

This article is republished with the kind permission of Leica Geosystesm and Vassar College.

Contact Marie-Caroline Rondeau, Leica Geosystems, marie-caroline.rondeau@leica-geosystems.com

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