What does UN WPP-adjusted population count and density mean?
GPWv4 consists of two types of population rasters: both types are based on census data, but one is adjusted to United Nations (UN) estimated national-level population counts. Both are of value; however, the UN WPP-Adjusted version has certain advantages and users may wish to use this type for regional and global analyses.
Grids consistent with national censuses and population registers
The Population Count and Population Density rasters were created using census and population data sourced from national statistics offices and other organizations. These rasters are consistent with the counts from national censuses and population registers and maintain fidelity to the input population data. Essentially, these are rasters of raw census data and are of use to those interested in working with native population data.
UN WPP-adjusted grids
The UN WPP-Adjusted Population Count and UN WPP-Adjusted Population Density rasters are based on counts from national censuses and population registers, but are adjusted to match official UN population estimates. As a result, these rasters will have greater consistency across countries for regional or global analyses.
In some countries, official demographic statistics are affected by issues including errors in reporting, lack of timeliness, or incompleteness in coverage. The UN Population Division takes great care in adjusting national census data based on these issues. They have created the UN World Population Prospects (WPP): The 2015 Revision, which includes the current official national-level UN population estimates that take into account issues in the official census data.
GPWv4 therefore provides data sets that adjust population estimates to the UN WPP in order to correct for any issues in the figures reported by national statistics offices. For each country, a single, national-level adjustment factor between the UN WPP estimates and the GPWv4 estimates is calculated for 2000, 2005, 2010, 2015, and 2020, and applied to each sub-national unit to calculate UN WPP-adjusted population estimates.
Please see the GPWv4 Methods web page for additional information on the formulas used to adjust the population estimates to the UN national estimates. Please see the UN Department of Economic and Social Affairs (DESA) web page for more information on the 2015 Revision of World Population Prospects.
Comparison to GPWv3 data notation
The Population Count and Population Density rasters are equivalent to the GPWv3 grids with filenames ending in “…g”, while the UN WPP-Adjusted Population Count and UN WPP-Adjusted Population Density rasters are equivalent to the GPWv3 grids with filenames ending in “…ag”.