The French expansion and domination in Italy between the Revolutionary Age and the Empire based on a widespread activity of territorial knowledge, which rested in the Corps of Engineers-Geographers and in the Military Genius the main actors. The paper summarizes the results of long research on this activity, carried out in the islands of Elba (Tuscany) and Palmaria (Liguria): two strategic islands in the western Mediterranean. The need to equip the territories dominated by the French with increasingly functional defenses, gave a strong impulse to the renewal of surveying and cartography, with the use of geodetic projections, views and three-dimensional models. Elba example is significant for the complete triangulation of the island connected to the Corsica one (with part of Sardinia and the smaller islands of the Tuscan archipelago). Geographer engineers such as Tranchot, Simonel, Moynet, Puissant worked on these activities and produced some maps and a small model of part of Elba. In the Palmaria example the three-dimensional reproduction (plan-relief) was contextual to the work of Genius engineers who produced a vast and organic corpus of maps of various scales, views, sketches and watercolors, suitable to represent the most complete visualization of the landscapes where to insert defensive buildings. The collaboration between French and Italian engineers took advantage of this first experience in designing some batteries. However, it was the post-Napoleonic decades that made Palmaria island a powerful “fortress island” to defend the entrance to the Gulf of La Spezia, where the military arsenal (commissioned by Cavour and built by Domenico Chiodo) arose.