Spatiotemporal Network Evolution of Anthropogenic Night Light 1992-2015
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Satellite imaging of night light provides a global record of lighted development from 1992 to present. Not all settlements can be detected with night light, but the continuum of built environments where a rapidly growing majority of the population lives generally can. Segmenting a continuous field into discrete spatially contiguous subsets of pixels produces a spatial network in which each contiguous segment represents a distinct network component. Representing the continuum of lighted development as spatial networks of varying spatial connectivity allows the generative conditions for power law network structure to be applied in a spatial context to provide a general explanation for similar scaling observed in settlements and other land cover types. This study introduces a novel methodology to combine complementary sources of satellite-derived night light observations to quantify the evolution of the global spatial network structure of lighted development from 1992 to 2015. Area-perimeter distributions of network components show multifractal scaling with larger components becoming increasingly tortuous. Rank-size distributions of network components are linear and well described by power laws with exponents within 0.08 of -1 for all 27 subsets of geography, year and degree of connectivity - indicating robust scaling properties. Area distributions of luminance within network components show an abrupt transition from continuously low-skewed for smaller components to discontinuous, nearly uniform luminance distributions for larger components, suggesting a fundamental change in network structure despite a continuum of size and shape. These results suggest that city size scaling, observed inconsistently for administratively-defined city populations, is more consistent for physically-defined settlement network area and can be explained more simply by a spatial network growth process.
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