Precise 3D Underground Modeling Ensures Next Chapter of Storied Tunnel in France
To produce a 3D model for future repairs for the tunnel under their historic railway station, the government of Metz, France hired surveyor firm A.L.I.D.A.D.E.S. to capture all the data they would need in less than 48 hours.
In April 2022, the Eurometropole of Metz, France commissioned surveyor firm A.L.I.D.A.D.E.S. to carry out a topographical analysis of the road tunnel passing under the esplanade of the central station of the city, in order to produce a 3D model for future repairs.
In operation since 1908 and built in a neo-Romanesque style, this grandiose railway station building was intended to be a symbol of the power of Emperor William II over Metz and the surrounding region, which was annexed in 1871 by the German Reich and returned to France under the Treaty of Versailles in 1919.
The Metz train station: a historic monument and one of the longest train stations in Europe © Franck Henry
At more than 300 meters, it is still one of the longest train stations in Europe. Classified as a historic monument in 1975, it has been voted several times the most beautiful station in France, in part because of its forecourt is lighted by elegant sunflower lamp posts, created in the early 1990s by the famous designer Philippe Starck, that automatically rotate in the evening to diffuse their light.
In 2020, surveyors Paul Chavant and Julien Pierson created a new firm called A.L.I.D.A.D.E.S., a French acronym standing for surveying, subdivision, implantation, delimitation, monitoring, division, condominiums outline, and 3D scanning. Based in Metz and Thionville in Lorraine, in Strasbourg and Enztheim in Alsace, the firm’s team of 20 specializes in the acquisition, understanding and processing of 3D data.
Today, A.L.I.D.A.D.E.S uses ten Trimble total stations, including two Trimble® SX10 hybrids (total station with 3D scanner), three Trimble® S7s, four Trimble® S6s, one Trimble® S3, one Trimble® X7 scanner, five Trimble GNSS receivers (one R12i, two R10s, two R8Ss, and two TSC7 data collectors), all processed through Trimble Business Center (TBC).
The SX10 scanning total station positioned at the entrance of the underpass © A.L.I.D.A.D.E.S
“The main reason for choosing Trimble, in addition to it being a global brand, is the simplicity of use and configuration of these systems, and the fact that it is very easy to bring together recent and old equipment without too many compatibility concerns,” Pierson said.
Thanks to technologies like frictionless drive, active prism, and video imaging, he believes that Trimble’s topographic stations are the best on the market.
On the GNSS side, Trimble's recent technological advances, especially Trimble Pro Point™️ technology and the Trimble TIP™️ IMU tilt compensation technology on the Trimble R12i receiver, have also changed the firm’s business.
In line with the new technologies of its business, the young company is focused on solutions for precision. “We have been using Trimble technologies for a long time, including in previous work at Geotopo, a Trimble distributor in France,” Pierson said.
With tunnel traffic closed for only 48 hours, A.L.I.D.A.D.E.S. had to work fast to carry out its survey of the 400-meter underground passage, including a 300-meter curve.
For this mission, Chavant and Pierson were assisted by their young technician, Hugo Mazoye. Another challenge was to understand and perform the tunnel survey with external control points only at the entrance and exit.
“We also had to acquire a precise georeferenced point cloud taking into account linear alteration in the field and in the office” said Pierson. “To meet so many requirements, we quite naturally opted for the combined use of our two Trimble SX10s and our X7 static scanner before using Trimble Business Center.”
A.L.I.D.A.D.E.S. proceeded conventionally via successive surveys on spherical prisms in a georeferenced coordinate system to obtain a faithful image without any problem of control points in the tunnel. “The SX10 station coupled with the “magic” (in terms of visibility) screen of the TSC7 controller, allowed us to create a global cloud with a station-to-station referencing based on a precision polygonal. This point cloud became our reference for the rest of the study, without us needing to perform subsequent resettings.”
A.L.I.D.A.D.E.S. then chose to densify the point cloud using the Trimble X7. “This scanner allows quality acquisitions to be carried out in a rhythmic manner that’s easy for the operator,” Chavant said.
To facilitate the cloud-to-cloud registration and to obtain a regular frame, the team still set some registration targets within the tunnel. In the end, 11 different positions with the SX10 scanning total stations were carried out and no less than 130 positions with the X7 scanner. The traverse closure of landmark routing was about 2.5 mm, an extremely accurate result.
Back in the office and thanks to the TBC software, A.L.I.D.A.D.E.S. easily calibrated the data from the two point clouds to deliver a dense georeferenced point cloud, as well as a complete topographic plan.
Point cloud classifying the entire tunnel © A.L.I.D.A.D.E.S
“The main advantage of Trimble scanners is obtaining a complete image of the structure at a given moment that avoids needing to return to the field,” said Chavant.
”Trimble Business Center allows us to process points clouds acquired in a georeferenced GPS coordinate system such as RGF93 CC49 in complete safety while preserving any possible parameters of linear alterations associated with these systems.”
In the end, in addition to a cloud of dense points georeferenced in the RGF93 CC49 system, A.L.I.D.A.D.E.S. also delivered a complete 2D topographic plan of the site, elements which will be integrated into the Eurometropole database for the subsequent production of a 3D model that will be used to plan a future repair of the underpass.
Point cloud merging data from SX10 and X7 scans and used as a basis for precision work © A.L.I.D.A.D.E.S
Bonus: Testing the new Trimble® X12 3D scanner
At the request of Geotopo, an authorized Trimble distributor, the A.L.I.D.A.D.E.S. team recently tested the Trimble X12 3D scanner on a bridge.
Pierson's verdict: “This device is breathtaking in terms of its acquisition quality, its density of points and its disconcerting ease of use. Its performance is up to what we expected for large engineering structures, as well as for small interior surveys.”
He concluded, “The collaboration with Z+F is a smart choice and reinforces the critical analysis of Trimble engineers. With its design, precision, adaptability and live registration on the cloud-to-cloud terrain, it is undoubtedly one of the most successful products of its generation. We can therefore bet that the Trimble X12 3D scanner will very soon become a new benchmark in the market.”
Learn more about Trimble S7 total station and Trimble Business Center software.