Occitanie: Greater Nîmes, Sète and Grau-du-Roi mapped in 3D by IGN, an incredible spectacle.

A dozen planes have been criss-crossing France since 2020 to capture its 3D footprint using Lidar technology: buildings, vegetation, open spaces and coastlines. The Occitanie region was among the first regions flown over and it is a vast area around Nîmes which acts as a showcase for the project.

Loïc Gondol is a cartographic engineer and head of the “High Density Lidar (HD)” mapping project for the National Institute for Geographic and Forest Information (IGN). As part of the deployment of a new mapping technology, he and his team chose to highlight a 50km block of data with Nîmes at its center. In smaller areas, the ports of Sète, Grau-du-Roi, and the towns of Uzès, Arles, Sommières and Combs are also available. Loïc Gondol recounts this pharaonic project.

What is Lidar HD, the technology used to map France in 3D?

The Lidar devices are cubic boxes with an edge of approximately 70 cm, embarked in small specially equipped planes. The device sends LASER beams towards the ground and the round trip time of the LASER beam reflected by the first obstacle that it will encounter determines the position of a three-dimensional point in space. The aircraft flies back and forth in a straight line and records data on a strip 300 meters wide.

We aim for an average of 10 measurement points per square meter, on open ground. Where there is forest and vegetation you can actually have an even higher density. In fact we do not choose completely, we only calibrate the device to have a minimum density according to the needs of the project.

The scanner is on board a small plane whose cabin is specially equipped.
IGN

Why did you choose Nîmes and its surroundings as a showcase?

We had hesitated between the blocks of Nîmes or Montpellier, which had been the first to be available. We opted for the region of Nîmes, in view of the buildings known to the general public (Pont du Gard, arenas).

What happens after the data collection stage?

There are several steps. The data is downloaded, checked, and finally processed to classify the points by type: ground, buildings, vegetation, etc. We are looking to put artificial intelligence behind it to automate this step even more. We can then produce digital models (kinds of 3D maps) finely representing the surface of the ground. Moreover, by superimposing a point with aerial images, it can then be colored.

Finally, the data is made freely accessible. We started to collect them last spring, the processing of the first data sets will be finished at the end of 2022. So for the moment, it takes 18 to 24 months for all the stages. Eventually we hope to reduce to a big year… Let’s say 15 months maximum. It’s a job that occupies 80 people at IGN between the different stages, and between 200 and 250 people with subcontractors.

The project plans to map the whole of France, for the moment, the areas already scanned whose data have been validated only cover part of Gard, Hérault, Lozère and part of the PACA coast. Why ?

For the moment, we only have about twenty square kilometers really cleaned and classified in high density, but we already had a good part of the territory in low density (2 points/m² on average, Editor’s note). The priority of acquisition of one area or another is decided according to priorities issued by sponsors. Areas with a concentration of the highest natural risks (flood, fire, etc.) are part of these priorities.

Who are these sponsors?

We have funding from the public action transformation fund of the recovery plan of the Ministry of Agriculture, and risk prevention from the Ministry of Ecological Transition. The Occitanie region was one of the first funders, with around one million euros. Discussions are ongoing with other regions.

You have issued a call for projects to entrepreneurs, what kind of applications can you imagine for these new kinds of maps?

As I was saying earlier, we think above all about the prevention of natural risks. Soil modeling will simulate rising water levels to see where the water would go. This is a problem that Nîmes in particular knows well. The models are accurate to within 10 cm in altitude, that’s about the height of a sidewalk! There is material for start-ups to offer tools that can be used by design offices and public authorities. We all have to win.

Data credit: IGN®

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