Maps are estimates of economic activity within 5×5 kn grid cells, based on satellite observation of night lights.

Images are from the paper Mapping regional economic activity from night-time light satellite imagery (Doll, Muller and Morley, Ecol Econ 2006, doi:10.1016/j.ecolecon.2005.03.007).
From the abstract:

Night-time light remote sensing data has been shown to correlate with national-level figures of Gross Domestic Product (GDP). Night-time radiance data is analysed here along with regional economic productivity data for 11 European Union countries along with the United States at a number of sub-national levels. Night-time light imagery was found to correlate with Gross Regional Product (GRP) across a range of spatial scales.

Maps of economic activity at 5 km resolution were produced based on the derived relationships. To produce these maps, certain areas had to be excluded due to their anomalously high levels of economic activity for the amount of total radiance present. These areas were treated separately from other areas in the map. These results provide the first detailed examination of night-time light characteristics with respect to local economic activity and highlight issues, which should be considered when undertaking such analysis.