From 1839, Jean-Baptiste Bory de Saint-Vincent leads the topographical and geographical section of the commission in charge of the scientific exploration of Algeria. During three years, between 1840 and 1842, with a team of ingeniors he travels accross the country and therefore contributes to the collect of topographical, geographical and demographical information. The french government demands this data in order to manage the region. The information furnished by Bory de Saint-Vincent are nevertheless astonishing, because he depicts a huge territory, much bigger than in reality, which is almost empty with men. In a geographical and demographical point of view, he deforms the algerian colonial space and turn it into a spatial idea built upon his own illusions. Despite his isolation in the scientific french field and in the french government, Bory de Saint-Vincent remains straight in his conception. This article means to understand how he builds such a theory. In a perspective of spatial history of colonialism, it also intends to question the complex relations tied up between the production of geographical knowledge and the military and colonial context in which it takes place. The example of Bory de Saint-Vincent in Algeria invites indeed to proceed to a global analysis of the context, the discourse and the personal matters. It therefore leads to a systemic lecture of the production of scientific knowledge in colonial context.