In 2008, the world became mostly urban, when for the first time, more people lived in cities than rural areas. That year, we also crossed an important technological threshold – for the first time there were more mobile broadband Internet subscribers than fixed. A new book by Anthony Townsend, SMART CITIES: Big Data, Civic Hackers, and the Quest for a New Utopia, explores the intersection of these historic shifts. With the UN projecting that 90 percent of population growth in coming decades will occur in cities throughout the developing world, new solutions are needed to address the rapid expansion of informal settlements. In this excerpt, Townsend explains how a new volunteer effort Map Kibera is combining consumer technologies and open source GIS to chart one of Africa’s largest and most notorious slums.