Cergy Pontoise is developing digital twins in its territory

A former new town, Cergy-Pontoise became an urban community in 2004. Straddling two departments, Yvelines and Val d’Oise, it covers an area of ​​80 km² and has more than 210,000 inhabitants spread over 13 municipalities. of varying size, ranging from more than 65,000 inhabitants to less than 600. Cergy-Pontoise also hosts nearly 30,000 students and has a few major stations, such as Cergy Préfecture. For several years, the agglomeration has been working on the implementation of three-dimensional representations of its territory around its historical geographic information system (GIS), provided by Esri. Franck Touyaa, head of the geomatics department of the Cergy-Pontoise agglomeration community, explains how these digital twins of the territory are developed, what challenges they meet and what are the points of vigilance during such an approach.

“The evolution of the territory is a real question”, launches Franck Touyaa. “It raises issues of mobility, housing and occupation of space. To address these issues, the agglomeration relies in particular on its GIS, set up in 2000, which provides support for decisions thanks to the representation of the territory. This GIS is managed by the geomatics service of Cergy-Pontoise, which is attached to the urban strategy and solidarity department and employs seven agents. “GIS has always relied on solutions from Esri. It started with the consultation of data from the cadastre and urban plans, then over time it integrated data on all the skills of urban development: roads, green spaces, networks, culture, sport, etc. “, relates Franck Touyaa. In 2019, Esri’s solution incorporated new data and display methods that made it possible to acquire and visualize 3D data. “Since the 80s, we had physical 3D models. We then started to think about the use of 3D for certain sectors, which are difficult to visualize in two dimensions,” recalls Franck Touyaa. The evolution of Web technologies, by facilitating access to three-dimensional data, made it possible to meet several business needs, mainly around urban planning. “In the Grand Center Coeur d’agglo district, for example, we have a slab town plan, with several overlapping levels”, illustrates Franck Touyaa.

Acquire precise data on the territory

Cergy-Pontoise then began the development of a multi-scale 3D platform, intended to become a digital twin of the agglomeration, based on modeling technologies such as BIM (building information modeling) and its equivalents for cities and territories. “At the start of the project, we worked a lot with Esri. We participate in their Benefits360 support program, with a project manager who knows our issues well and offers us possible solutions. To familiarize ourselves with the handling of 3D tools, we did a Proof of Concept (PoC) on the Grand Center model,” says Franck Touyaa.

A 3D scan view of part of the territory of the Cergy-Pontoise conurbation.

To acquire the data, several steps were implemented. The agglomeration already had aerial shots taken in 2018 (orthophotos and obliques for the facades). In 2019, she also conducted Lidar scans, which provide very precise point clouds. Thanks to this data, all the buildings could be generated in 3D. Finally, Cergy-Pontoise also used the 3D scanning technique on larger sectors. “We went on acquisitions as close as possible to the ground, with a surveyor who used a drone for 3D scanning on the slab, but also below, also capturing networks, doors and other objects”, describes Franck Touyaa . According to the latter, data acquisition is not what takes the most time. For the aerial shots in 2018 and the Lidar in 2019, the longest part was obtaining flight authorizations. In the case of the Grand Center district, a few days were enough for the drone flight. Then it’s done in a day. “The longest part is the restitution with the surveyors, to redraw the BIM (building information modeling) models. For the project, this was done in two stages, in 2020 and 2021, each time with four months of fieldwork and processing,” says Franck Touyaa.

Enrich 3D models with other data

The representation of space and territory in 3D requires more time. “We have almost finished after a year of work, we still have a few checks to carry out”, specifies Franck Touyaa. Now, the geomatics service manages the platform itself, with one agent in particular who is in charge of 3D. The team now wants to continue the process. “The goal is not to have one digital twin, but several. We have one for the Grand Center district, another with the entire territory in lower quality, generated from aerial shots. The aim is to offer data to the population and to integrate new sectors. The first area modeled, the Grand Center district, allowed us to understand the technologies. We are going to extend the idea to other themes and other urban projects”, indicates Franck Touyaa. Thanks to the work already carried out, the department is now able to offer businesses 2D or 3D visions depending on the subject. “With interactive maps and knowledge of the territory in 3D, we can find our bearings better than on a two-dimensional map. The added value is visual,” emphasizes Franck Touyaa. “We were able to use the model for simulation, in order to show our colleagues the value of positioning the buildings in their environment. For example, we made several simulations of a university building, with co-visibility colors to indicate what we could see of it from a particular point of view,” he continues. Another avenue being studied is the provision of open data, “but the raw data takes up space”, points out Franck Touyaa.

The 3D model of the Agglo Grand Center Coeur project.

Currently, the agglomeration is working on adding data to the 3D platform. “We are thinking about the integration of data flows. For example, we produced a version for the general public of the Grand Center model, which should not be dissociated from the ongoing development projects,” explains Franck Touyaa. Thus, the model mixes the existing buildings with those in the project. “The GIS must be included in the agglomeration’s communication systems, it is not a separate tool”, points out the manager. This integration should allow urban projects to interrogate a website, for example to retrieve a data stream on traffic information in real time. “We have already integrated into the Grand Center model the consultation of VélO2 self-service bike stations, which can be at the top or at the bottom of the slab”, illustrates Franck Touyaa. The service also uses all the data already present in the GIS, such as a database that contains 40,000 trees and their positions. “We are examining whether we can retrieve the height of these trees via Lidar, in order to improve our knowledge of them”, indicates the manager.

Think about the evolution of the model and the associated IT issues

For the head of the geomatics department, the transition to 3D modeling raises various issues, which should be considered upstream. The first is that of updating the data, in order to bring the models to life. It is necessary to plan business processes on the evolution of the model and the acquisition of new data. “We have several methods for the evolution of the model, such as recovering the BIM models of the buildings from the contracting authorities. It is a real reflection, global to all territories. When possible, it saves us time. Integrating BIM tools and GIS takes time and skill. However, in our case, the convergence between Esri and Autodesk solutions made things easier. I was surprised at the ease of integration,” says Franck Touyaa. A second approach, in high-stakes sectors, consists of calling on service providers specializing in drone acquisition. Finally, during the next aerial shots, the service will request a data update. Another issue relates to the knowledge that the professions have about the use of 3D. “Today almost all the professions in the conurbation know how to use GIS, but 3D has not yet entered the geomatics culture. We have a work of accompaniment and animation on the subject to carry out ”, estimates the person in charge.

Other issues relate more to the information system, such as the storage and weight of data, aspects that must be taken into account. 3D orthomapping represents several GB of data, it has a cost. We must ask ourselves the question upstream of what we are going to do with this data”, explains Franck Touyaa. For its GIS, Cergy-Pontoise hosts an internal Geo agglo portal. The ortho-3D data, which represents approximately 74 GB, is published on the SaaS service Arcgis Online from Esri. “Navigation in the model is done in streaming. The choice of the cloud was relevant to restore the data with correct access times: visualization does not require very powerful computers, a browser and a good connection are enough”, points out the manager. Finally, it is also necessary to ensure the interoperability of formats. “It was one of our requirements to have interoperable formats,” emphasizes Franck Touyaa. “The GIS is already well integrated with our business tools, such as the tree database, which can thus have several uses; but our ambition is to include more, why not in 3D,” he says in conclusion.