Spatial Analysis of Global Urban Extent from Night Lights

Previous studies of DMSP-OLS stable night lights have shown encouraging agreement between temporally stable lighted areas and various definitions of urban extent. However, these studies have also highlighted an inconsistent relationship between the actual lighted area and the boundaries of the urban areas considered. Applying detection frequency thresholds can reduce the spatial overextent of lighted area (“blooming”) but thresholding also attenuates large numbers of smaller lights and significantly reduces the information content of the night lights datasets. Spatial analysis of the widely used 1994/95 stable lights data and the newly released 1992/93 and 2000 stable lights datasets quantifies the tradeoff between blooming and attenuation of smaller lights. For the 92/93 and 2000 datasets, a 14% detection threshold significantly reduces blooming around large settlements without attenuating many individual small settlements. The corresponding threshold for the 94/95 dataset is 10%. Spatial analysis of light size distributions reveals that conurbations larger than 80 km diameter account for < 1% of all settlements observed but account for about half the total lighted area worldwide. Comparison of lighted area with built area estimates from Landsat imagery of 17 cities shows that lighted areas are consistently larger than even maximum estimates of built areas for almost all cities in every light dataset. Thresholds >90% can often reconcile lighted area with built area in the 94/95 dataset but there is not one threshold that works for a majority of the 17 cities considered. Comparison of lighted area with blooming extent for 10 lighted islands suggests a linear proportionality of 1.25 of lighted to built diameter and an additive bias of 2.7 km. While more extensive analyses are needed, a linear relationship would be consistent with a physical model for atmospheric scattering combined with a random geolocation error.