Mapping Decadal Change in Anthropogenic Night Light

Abstract

The Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) sensors have imaged emitted light from Earth's surface since the 1970's. Temporal overlap in the missions of 5 OLS sensors allows for intercalibration of the annual composites over the past 19 years [1]. The resulting image time series captures a spatiotemporal signature of human settlement growth and evolution. We use temporal Empirical Orthogonal Function (EOF) analysis to characterize and quantify patterns of temporal change in stable night light brightness and spatial extent since 1992. Temporal EOF analysis provides a statistical basis for representing spatially abundant temporal patterns in the image time series as uncorrelated vectors of brightness as a function of time from 1992 to 2009. The variance partition of the eigenvalue spectrum combined with temporal structure of the EOFs provides a basis for distinguishing between deterministic temporal trends and stochastic year to year variance. The low order EOFs and Principal Components (PC) space together discriminate both earlier (1990s) and later (2000s) increases and decreases in brightness. Inverse transformation of these low order dimensions reduces stochastic variance sufficiently so that tri-temporal composites depict deterministic decadal trends. The most pronounced changes occur in Asia. Throughout Asia a variety of different patterns of brightness increase are visible in tri-temporal brightness composites – as well as some conspicuous areas of apparently decreasing background luminance and, in many places, intermittent light suggesting development of infrastructure rather than persistently lighted development. Vicarious validation using higher resolution imagery reveals multiple phases of urban growth in several cities, numerous instances of highway construction, extensive terracing networks and hydroelectric dam construction [3]. Lights also allow us to quantify the size distribution and connectedness of different intensities of development. Over a wide range of brightnesses, size distributions of spatially contiguous lighted area are well-fit by power laws with exponents near -1 as predicted by Zipf's Law. However, the larger lighted segments are much larger than individual cities; they correspond to vast spatial networks of contiguous development.[2]