Following the devastating fire of 2019, geotechnical experts leaned on partners such as Trimble during the active reconstruction phase of Notre Dame Cathedral to combine structural integrity with historical accuracy.

The team that was assembled to complete the restoration of Notre Dame Cathedral, by the local public authority in charge of the conservation and restoration of the cathedral, 'Établissement Public Rebâtir Notre-Dame de Paris', faced a crucial challenge. Following the devastating fire, the conservation and restoration efforts needed to maintain the delicate balance of blending historical preservation with cutting-edge engineering. By leaning on partners such as Trimble, geotechnical experts ensured the structural integrity and historical accuracy of the beloved landmark throughout the entire active construction phase.

Precision in a challenging environment

During peak periods of the reconstruction project, the Notre Dame site was a hive of activity, with constant movement of personnel and heavy equipment. This dynamic environment, coupled with scaffolding and limited access, presented significant hurdles for traditional surveying. To meet these challenges, the Paris-based geotechnical and structural monitoring firm Spie batignolles ITM+ deployed Trimble S5 and S7 total stations. These instruments were strategically positioned to provide comprehensive, automated hourly measurements of high-risk structural zones, and they still remain in place since the restoration phase.

Key features such as autolock technology allow the total stations to reliably lock targets, despite obstructions and site movement, ensuring consistent, long-term measurements without manual intervention. Their high angular and distance accuracy has been crucial for monitoring minute displacements in such a sensitive historical structure. Furthermore, reflectorless measurement capabilities enable monitoring in hard-to-reach areas or on delicate surfaces where prisms cannot be installed.

Comprehensive data capture and analysis

Inside the cathedral, particularly in the roof structure and spire, cramped spaces and low light conditions have made manual inspections difficult and hazardous. The team is using the Trimble X9 3D laser scanner to gather crucial structural data in these challenging areas. Its ability to capture precise, real-time information in dim light and tight spaces means they can get a complete picture of the structure. The X9's protected mirror is especially valuable, keeping dust from interfering with its operation. Its small size also helps speed up their work, reducing the time personnel spent on the job site. Another draw was the X9’s silent operations. Many scanners on the market create sound while operating, due to the rotation of the mirror inside the machine. With its mirror protected, the X9’s quiet operating mode is particularly important in the cathedral setting.

For highly accurate manual measurements and establishing geodetic control, the team turned to the Trimble S9 robotic total station. Each month, it is used to carefully monitor critical elements such as the roof’s timber framework and the spire, allowing the team to track even tiny deformations as the wood dried and stabilized. The Trimble S9’s sub-millimetre accuracy is also vital for precisely locating structural targets and creating a highly accurate control network, ensuring that all 3D point clouds are aligned with millimetre-level precision.

A unified digital workflow

The data collected by Trimble hardware is integrated into a web platform known as ARGOS, owned by Spie batignolles ITM+. This has two associated modules: ARGOS Theo, which controls the total stations, and ARGOS Adjust for processing and validating the measurement data. This web platform provides real-time data visualization and reporting, ensuring all stakeholders have access to the latest structural health information.

Beyond these custom tools, Trimble RealWorks processes the X9 laser scan data, performing cloud-to-cloud comparisons and quantifying structural deformations. Trimble Access enhances field efficiency for manual measurements and control point surveying, while Trimble Perspective provides real-time control for the X9 scanners on-site.

While the Spie batignolles ITM+ team is focused on monitoring and geodetic surveying, their data is fed into a broader digital ecosystem supporting architects, engineers and restoration teams. This comprehensive approach ensures the meticulous and safe reconstruction of Notre Dame, safeguarding its legacy for future generations.