Spatial analysis of global urban extent from DMSP-OLS 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 1994/1995 stable lights data and the newly released 1992/1993 and 2000 stable lights datasets quantifies the tradeoff between blooming and attenuation of smaller lights. For the 1992/1993 and 2000 datasets, a 14% detection threshold significantly reduces blooming around large settlements without attenuating many individual small settlements. The corresponding threshold for the 1994/1995 dataset is 10%. The size–frequency distributions of each dataset retain consistent shapes for increasing thresholds while the size–area distributions suggest a quasi-uniform distribution of lighted area with individual settlement size between 10 and 1000 km equivalent diameter. Conurbations larger than 80 km diameter account for < 1% of all settlements observed but account for about half the total lighted area worldwide. Area/perimeter distributions indicate that conurbations become increasingly dendritic as they grow larger. 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 1994/1995 dataset but there is not one threshold that works for a majority of the 17 cities considered. Even 100% thresholds significantly overestimate built area for the 1992/1993 and 2000 datasets. 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. A Gaussian detection probability model is consistent with an observed sigmoid decrease of detection frequency for settlements < 10 km diameter. Taken together, these observations could provide the basis for a scale-dependent blooming correction procedure that simultaneously reduces geolocation error and scattering induced blooming.