An archivist, historian specialising in Roman Gaul and renowned numismatist, Auguste Longnon joined the Commission de Topographie des Gaules (CTG) in its final years. He was introduced to the world of scholarship by Alfred Maury and Anatole de Barthélemy. Passionate about toponymy and topography, his major works corresponded to the CTG’s interests, particularly his monumental Atlas historique de la France depuis César jusqu'à nos jours (1885).