According to many researchers, the map's didactic potential is often underestimated or even unknown in schools. Indeed, thematic map reading instruction is not systematic and does not go beyond locating geographic features and place name identification. However, thematic map reading is proven to be a relevant exercise which helps students to develop their critical mind and their geographic reasoning. This paper suggests a learning strategy for thematic map reading, based on Zgor's model, which aims to develop geographical thinking among secondary school students. It is illustrated by an exercise on population density in Tunisia.