Spatial gradients of urban land density and nighttime light intensity in 30 global megacities

The spatial agglomeration of urban elements results in the center-periphery urban structure, but the difference in spatial gradients of socioeconomic and physical elements is unclear. This study investigates how urban land density (ULD) and nighttime light intensity (NLI) decline with the distance to center(s) using the inverse-S function. Taking 30 global megacities as examples, we acquired their urban land and nighttime light in 2020 to represent urban physical and socioeconomic elements, respectively. ULD and NLI in concentric rings have been calculated to compare their spatial gradients from the city center(s). Results show that both ULD and NLI decrease slowly around city centers, followed by a relatively quick decline to suburban areas, and then decrease slowly again to a background level, showing an inverse-S shape. This spatial gradient can be well-fitted by the inverse-S function, whose parameters reflect disparities in urban extents and urban forms. NLI decreases faster than ULD, resulting in smaller radii (extents) of NLI, which shows the spatial agglomeration of socioeconomic elements is more obvious than that of physical space. This gap requires balanced development of socioeconomic and physical elements in megacities to avoid low-density urban sprawl and promote sustainable urban development.