Camptocamp has been working with its partners for over four years on the technical implementation of the NexSIS 18-112 project, the future information and command system for the French fire and rescue services. The project is being carried out on behalf of the Agence du Numérique de la Sécurité Civile (ANSC), which reports to the Ministry of the Interior. The Camptocamp teams, responsible for implementing the NexSIS geographical information system, faced several challenges in order to develop a modern, resilient and ergonomic solution. True to its DNA, Camptocamp is building this solution exclusively using open source building blocks. This desire to promote and contribute to open source tools wherever possible is shared with our customer. Geographical information has a central place within NexSIS. Most of the functions of NexSIS are supplied or enriched with geographical data via mainly cartographic user interfaces: rapid localization of an emergency situation following a call to 18 (or 112); transmission of intervention information to all the forces responsible for this emergency situation (fire department, police, gendarmerie, municipal authorities, prefectures, road or energy network operators, etc.); mobilization of the most appropriate personnel and equipment to be sent on rescue missions. All these functions require heterogeneous geographical data from very different sources: Data producers (Base Adresse Nationale, Institut Géographique National, HERE), organizations and companies (SNCF, RTE, Veolia, Voies Navigables de France, etc.), field reports (fire and rescue services), open data (OpenStreetMap), etc. This heterogenity leads to a certain technical complexity, but above all to an extensive set of functions in NexSIS, ultimately providing a better service for the end users, the firefighters. The aim of the presentation is therefore to use the example of NexSIS to show how open source tools and data can make the work of the emergency services easier, so that they can respond more quickly and efficiently to emergency situations and ultimately offer citizens a better service.