WASHINGTON -- Mexico City has been described as the hemisphere's most populous urban area but authorities differ on how to calculate its population and no one knows its exact size.
Various statisticians put the population of Mexico's capital city between 17 and 18 million and attribute the city's rapid growth to a steady influx of poor peasants from the surrounding countryside.
'I don't think anybody knows for sure (what the population is),' a State Department spokesman said. 'No one would be lying if he said 17 or 18 million. It's big!
'People keep coming in from the rural areas and settling, sometimes rather informally. I don't know of any exact figures.'
A Census Bureau official said 'there is no consensus on how to define an urban area' but the agency has devised its own system of determining the number of residents in foreign countries.
Using the system, the bureau projects Mexico City has 'an urban agglomeration population' of 16.9 million. Projections for July 1, 1985, show Mexico City probably will have a population of 20.2 million by 1990 and 27.9 million by the year 2000.
Alan Patera, geographer with the Bureau's Center for International Research, said this updated information is expected to be released in a new publication 'within the next month or so.'
A 1979 Census listed Mexico City as the second largest city in the world with 9.1 million people and the largest in the western hemisphere. Shanghai was listed as the largest in the world with 10.8 million.
Patera said various authorities use a variety of different methods to calculate population. He said the United Nations publishes information supplied by the Mexican government, which defines Mexico City as 'a federal district' and recognizes certain adjacent municipalities.
The United Nations 1983 Demographic Yearbook said population in the Mexico City 'urban agglomeration' was 14.7 million, while the city proper had only 9.1 million residents.
'We don't use this at all,' Patera said of the Mexican government's figures. 'We probably go out further than they do.'
Patera said that the bureau, in its system of calculating residents, includes people in its population counts as long as there are as many as 5,000 residents per square mile. For comparison, he said Hong Kong has 232,000 people per square mile and Bombay has 456,000 per square mile.
'While there is no consensus of how to define an urban area, we have produced a list based on consistent definitions which allows one to compare the urban areas throughout the world.'