The drawing of the contemporary city for 3 million inhabitants, elaborated by Le Corbusier and Pierre Jeanneret in 1922, in time, has become the canonic image of the project. The drawing was contained in a cylindrical screen to be observed by means of a device which Le Corbusier called Diorama. Based on primary source documents, a restitution hypothesis is proposed to deduce the geometric projection system used in the construction of the image. The method, apparently used in the curvilinear perspective is really a variant of the traditional system of canonic flat perspective.