09 April 2025
3D Georeferencing of a Collector in Nantes: A Challenge Met Without Water Discharge
Recently, our team took on a major challenge at Nantes Métropole: georeferencing a hard-to-access collector without having to interrupt its operation. A look back at a mission combining cutting-edge technology, operational efficiency, and tangible benefits for infrastructure managers.
An Accessibility Challenge to Overcome
The project focused on the Lamoricière collector as part of a project to separate the sector. This type of work requires a detailed understanding of the existing infrastructure: condition of the structures, precise location of connections, profile in length, and exact measurements of the sections.
Two networks were identified:
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– EP 170/100: 223 linear meters, 18 connections, low water level (10 cm)
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– UN 230/150: 335 linear meters, 45 connections, high water level (80 cm)
The goal: to achieve precision georeferencing (Class A), without interrupting the water flow, incorporating all the necessary information to size the future network.
An Innovative Methodology Combining Photogrammetry and 3D Scanning
Faced with accessibility challenges, particularly for the UN 230/150 section, the INFRANEO team deployed a three-pronged approach:
On-site inspection adapted to the environment
- – For the EP structure: pedestrian inspection with a camera, and 3D modeling via photogrammetry.
- – For the UN structure: raft equipped with 3 GoPro cameras to capture submerged volumes.
- – On the surface: 3D scanner to model the shafts, chimneys, and surrounding environment.
This approach allows us to cover all volumes, from the collector to the access points, ensuring redundancy of markings to guarantee model accuracy.
Data Processing in Several Stages
The field survey is complemented by rigorous computer processing:
- – Photogrammetry (Pix4D): assembling images to generate a dense point cloud.
- – Alignment and georeferencing by importing GPS points and matching them to physical markers.
- – Fusion of models from the surface, chimneys, and the collector to create a coherent global model.
- – Trimble Realworks software for densification, manual alignment, and technical exploitation.
Exploitation of Results for the Project
Thanks to the obtained model, our drafters can produce:
- – Longitudinal profiles
- – Precise plans of the structure
- – A virtual tour from a textured point cloud
Concrete Results and Decisive Advantages
This mission demonstrates the effectiveness of digital tools in the asset management of networks:
- – Class A georeferencing, including for connections and crossing elements.
- – Centimetric precision of section measurements.
- – Integrated TV inspection for a complete visual diagnosis.
- – Rapid intervention: 1 day for 355 meters of work on a raft, and half a day for the surface.
But most importantly, this approach avoids any water discharge, a significant time and cost-saving for project owners.
A Technology Serving the Public Interest
In Nantes and beyond, this expertise addresses a major issue: enabling local authorities to better understand, maintain, and rehabilitate their sewer networks. By leveraging 3D modeling, photogrammetry, and advanced topographic processing tools, INFRANEO provides a concrete solution to the challenges of tomorrow.
