This article seeks to question the way in which military officers in charge of cartographic work in Algeria around 1830 and 1840 dealt with information gathered from indigenous people. It shows that this information was considered indispensable and necessary, but that its collection was often done at random. Communication, language and transcription problems were also accentuated, in this context of French conquest, by resistance on the part of the interviewed population that the cartographers identified with more or less sensitivity. Despite its evident usefulness, indigenous knowledge was often considered in deprecative terms. This gap between usage and recognition sheds light on two rationalities at conflict. The cartographic work undertaken by the French, who tried to translate travel accounts into maps, reveals the accommodations in practice in the construction of a colonial space.