Base station locations. Base station locations on a conventional longitude-latitude projection (left) and a cartogram where areas are rescaled to be proportional to the number of inhabitants (right). The colours of the dots indicate the number of outgoing calls at each base station. The boundaries of subprefectures are shown for ease of orientation. The geographic distribution of the base stations are largely explained by the heterogeneous population distribution so that the point pattern appears less clustered on the right than on the left. Still, regions of significantly higher per-capita base station density remain (see Figure 2), especially in Abidjan, where even on the cartogram the dots are noticeably aggregated. The colours of the dots do not exhibit any clearly visible large-scale trends. However, a more careful statistical analysis shows that a significant correlation between traffic at nearby base stations exists (see Figure 3b).