The Global Population Database (GPOPDB) is used to map the populations of many countries around the world. The database uses a rectangular grid for mapping of 20 minutes of latitude by 30 minutes of longitude (about 2,000 square kilometers at the equator). For urban areas of more than 25,000 people, density circles are drawn to provide more detail.
Population data is primarily obtained through censuses, surveys and maps from the Bureau of the Census Library or from the countries themselves. Detailed data are obtained by working from the national totals towards the first-order administrative levels; typically state, province or region.