The use of a very detailed international boundaries template for, in some cases, relatively coarse resolution data is somewhat misleading, but was required to ensure a close match between the national coverages. For a few countries very detailed boundary data were available for which the spatial referencing information was not known. In the absence of better data, these were nevertheless incorporated in order to achieve maximum resolution. Yet, the ad hoc transformation, projection change and rubbersheeting required to make these data compatible with the DCW template have no doubt introduced positional error which may well reach a magnitude in the order of 1-2 km. The estimation method and a discussion of data accuracy are the subject of the following section.
Data sources vary by country.
In general we attempted to obtain population data from each census that has been carried out at the geographical level for which boundary data were available. Where official population estimates or projections were available from the national statistical office, these were used as well. Copies of official census publications have been collected over the past five years from a number of university libraries as well as the United Nation's Statistical Library and the U.S. Library of Congress. Additional material was available from the comprehensive holdings of the International Programs Center's library at the U.S. Census Bureau. Finally, any population figures published in yearbooks, gazetteers, area handbooks or other country studies have been used to ensure that population figures for as many time periods as possible could be used as the basis of the estimation.