Defining, measuring urbanization

How one measures the population of cities depends on what counts as part of the city, as the graphic below illustrates. Also, this doesn’t include larger urbanized areas where one city runs into another. An example of this is the Lagos-Accra corridor along the southern coast of Western Africa, which has perhaps 20 million people living along it (Lagos itself shows up on the Metropolitan section of the chart below, but how many additional people live in this corridor?). These cities, home to half the world’s population, are obviously challenging for the general idea of reaching unreached people groups, as in many of these cities the groups start to get assimilated in various ways.