Spatial and temporal characteristics of available census data. a) Year of the last national census data available (data source: GeoHive ) and b) average spatial resolution (ASR) of census data used in the construction of Gridded Population of the World version 3 (GPW3) and the Global Rural Urban Mapping Project (GRUMP). The ASR measures the effective resolution of administrative units in kilometers. It is calculated as the square root of the land area divided by the number of administrative units . It can be thought of as the "cell size" if all units in a country were square and of equal size.