population of urban agglomerations over historical time periods is a
major challenge due to the complexity of the urban growth process.
Villages can become towns, towns can grow into cities, and cities
can be transformed into urban agglomerations in a number of ways:
They may increase due to natural population growth - that is as a
result of a larger number of births than deaths; they may grow due
to rural-urban or urban-urban migration; or they may emerge as a
result of administrative changes. These administrative changes can
also involve several different processes: They may include the
incorporation of sub-urban areas or neighboring towns into a
larger city or the foundation of a completely new city - as
was the case with several newly established national capitals.
Administrative changes also include the re-naming of urban
agglomerations - particularly those that had foreign names been
assigned during the colonial period.
The figure above tries
to highlight some of these processes. The challenge of estimating
the population of human settlements arises from the fact that both
their name and their physical extend may have changed over past
decades - sometimes several times. Different towns (with different
names) may have been combined into one urban agglomeration - with
the name of one of the original towns or with a completely new name.
Population statistics referring to a "city proper" may be
discontinued and replaced by data for the much larger area of an
"urban agglomeration" with the same name - which may, or may not,
include other neighboring cities.
In the World
Urbanization Prospects we make an effort to prepare
consistent population estimates over historical time periods for
human settlement that follow the same definition. This often
requires that historical population estimates have to be re-adjusted
to a current definition that may not necessarily coincide with other
popular definitions for that city. The definition used in the
World Urbanization Prospects is documented
for each city in a database of