This is adapted from a note been sent today to a correspondent seeking clarification of our population estimate for the Dhaka urban area.
The population estimate that you referenced (15.4 million in 2012) was based upon the 2011 figure reported to the United Nations by the government of Bangladesh. We are pleased to provide clarification.
We will not be using that estimate as a basis in the future, because 2011 census data made it clear that the government’s geographical urban area definition is different than ours. Our analysis of the 2011 census yields a base of approximately 13.6 million, from which a 14.0 million 2012 estimate is derived (using the UN projected population growth rate). A new estimate for 2013 will be published in the 9th Annual Demographia World Urban Areas, which is scheduled for publication in the Spring of 2013.
Urban areas are areas of continuous development (as defined in urban analysis).Thus, an urban area conforms largely to the area of uninterrupted light that would be visible from a high flying airplane on a clear night. Urban areas are one of two dimensions of the city — its physical definition. An urban area contains no rural land. As a result, urban areas do not conform to administrative boundaries (such as the Dhaka Zila or any component of the Dhaka City corporations). Moreover, they are not metropolitan areas (which is the second dimension of the city — the functional definition of a city. This is the labor market area, which includes economically connected rural areas)
In the case of Dhaka, our urban area includes only the continuously developed parts of Dhaka and Narayanganj Zilas as well as the southern part of Gazipur Zila. This means, for example, that much of Dhaka Zila to the west of the river is not a part of the urban area. This also means that nearby Baipayl is not a part of the urban area, because there is rural land between it and Dhaka. The much smaller land area included in the urban area also illustrates the fact that the rural areas of the zilas are not included.
It appear that the Bangladesh government’s urban area population was really more akin to that of the metropolitan area. Regrettably, there are no international standards for definition of metropolitan areas, which makes comparisons risky.
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Note: Demographia expresses deepest condolences to the people of Dhaka and especially to families who lost loved ones in the tragic Tazreen Garment Factory fire in the Ashulia area overnight. And, may the injured be speedily restored to full health.
Wendell Cox (2012.11.25)
Demographia Observations 