Global Resolution Map

GPW v2 provides significantly higher resolution than initial version of GPW. It uses over 125,000 administrative units as compared to about 15,000 units in the initial version. To estimate how well the administrative units cover a country given the country area, a variable called "resolution" is introduced. The average resolution can be thought of as the "cell size" if all units in a country were square and of equal size. It is calculated as follows:

Resolution varies by country. The table below shows the countries with the highest and the lowest available resolution (ignoring very small countries and areas, most of which consist of only one administrative unit). Resolution is to some extent determined by the geographic size and average population density of a country. That is, smaller countries have a relatively higher resolution even before adjusting for the number of administrative units. Some of the highest resolution countries are relatively small (e.g., Luxembourg, El Salvador) and administrative units are generally necessitated by the presence of relatively densely-distributed populations (e.g., India, Netherlands). Among the countries with the lowest resolution, some include vast, largely uninhabited areas, where administrative units tend to be very large (e.g., Greenland, Libya). For other countries in this list, higher-resolution administrative units boundaries were simply not available for this project (e.g., Bosnia, Iran). Some are a combination of each problem (e.g., Chad, Egypt). The variation in mean resolution depends considerably on combination of geographic and demographic characteristics of the given and thus are not always comparable. For example, level-three administrative units in Canada can vary from a city-district to large tracts of uninhabited land whereas the same level in the continental United States varies much less in area. In general, although with the caveats noted above, it is accepted that the higher the resolution, the higher quality coverage.

The map above shows the mean resolution per country. The dark color represents higher value for resolution (i.e., poorer coverage). The highest value is for Greenland, at 380 per km, with only 15 large administrative areas. At the ends of the spectrum, low values (i.e., relatively good coverage) for the mean resolution are observed for Western Europe and the USA and the highest values (i.e., relatively poor coverage) tend to be found in Africa. The median global value for resolution is 40. To demonstrate the significance of higher resolution inputs, take the example of the United States: The mean resolution for is about 13, where level-three administrative data (i.e., tract) were used; had level-two administrative data (i.e., county) been used, the mean resolution would be about 55. The mean value for each country can be found in the Table of Summary Information.