Abstract
By 2050, half of the net increase in the world’s population is expected to reside in sub-Saharan Africa (SSA), driving high urbanization rates and drastic land cover changes. However, the data-scarce environment of SSA limits our understanding of the urban dynamics in the region. In this context, Earth Observation (EO) is an opportunity to gather accurate and up-to-date spatial information on urban extents. During the last decade, the adoption of open-access policies by major EO programs (CBERS, Landsat, Sentinel) has allowed the production of several global high resolution (10–30 m) maps of human settlements. However, mapping accuracies in SSA are usually lower, limited by the lack of reference datasets to support the training and the validation of the classification models. Here we propose a mapping approach based on multi-sensor satellite imagery (Landsat, Sentinel-1, Envisat, ERS) and volunteered geographic information (OpenStreetMap) to solve the challenges of urban remote sensing in SSA. The proposed mapping approach is assessed in 17 case studies for an average F1-score of 0.93, and applied in 45 urban areas of SSA to produce a dataset of urban expansion from 1995 to 2015. Across the case studies, built-up areas averaged a compound annual growth rate of 5.5% between 1995 and 2015. The comparison with local population dynamics reveals the heterogeneity of urban dynamics in SSA. Overall, population densities in built-up areas are decreasing. However, the impact of population growth on urban expansion differs depending on the size of the urban area and its income class.
Highlights
According to the latest predictions of the United Nations, the world population will increase from 7.7 billion in 2019 to 9.7 billion in 2050
Multi-sensor data fusion allowed better classification performances in arid and semiarid regions, where moderate resolution optical sensors suffer from the spectral confusion between bare soil and built-up areas
In case studies located in an arid climate (Gao, Saint-Louis, Windhoek, and Tulear), the total surface of the detected built-up areas was
Summary
According to the latest predictions of the United Nations, the world population will increase from 7.7 billion in 2019 to 9.7 billion in 2050. More than half of this global increase is expected to take place in sub-Saharan Africa (SSA) [1], driving high urbanization rates. The urban population of sub-Saharan Africa is expected to increase by 196% [2]. Such a rapid urbanization has already been experienced in the past by other countries during the first half of the 20th century [3]. A major difference from the experience of Europe or the United States is that the urbanization of SSA is occurring at low levels of per capita income, and in countries which are vulnerable to the global economy [3]. Urbanization is still one of the primary driver of habitat
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