In August 1502, the fifty-year-old Leonardo was appointed military architect and engineer to Cesare Borgia, son of Pope Alexander VI  and Marshal of the Papal Troops. Over the next few months Leonardo surveyed Borgia’s strongholds to the north and east of Florence, and created his most impressive surviving map, of the town of Imola.

On his return to Florence in 1503 Leonardo continued to make maps. Some were presumably commissioned by the Florentine government, such as surveys of the Valdichiana in southern Tuscany, and of the river Arno near Florence. Others were for Leonardo’s own pet projects, notably his idea of constructing a canal to bypass the Arno, or simply expressions of his interest in the formation of rivers and the weathering of mountains.