Night-time Lights Can Help Illuminate Trends in Urbanization
The map shown above portrays urbanization in fast-growing regions of India and Pakistan. Comparing the two maps, we can see both increasing density in urban centers and the expansion of urban areas. The map was produced as part of a project with the World Bank to assess changes in spatial extents of south Asian cities that had populations greater than 100,000 in the year 2000. We used a composite of data collected for the year 1999 and for 2010, which was derived from satellite imagery of lights at night, as observed by the Defense Meteorological Satellite Program’s Operational Linescan System (DMSP-OLS).
Areas are designated as urban when the intensity of light visible at night exceeds the threshold of 13 DN (digital number). The threshold of 13 DN was chosen based on statistical analysis of the distribution of values and on prior published work. The original satellite imagery picks up many different lights—e.g., automobile lights, flaring of natural gas wells, wildfires, and small settlements—but the threshold filters out areas not considered urban. On the map for 1999, all the isolated colored areas correspond to cities with a population count of at least 100,000 people. By 2010, we can see that the cities are so close together that their nighttime lights form a continuous urban corridor, or agglomeration, between Delhi and Lahore. This urban agglomeration encompasses 67 cities above the 100,000 population threshold.
This comparison between 1999 and 2010 urbanization was made possible by a new version of the DMSP-OLS Nighttime Lights data developed by Chris Elvidge of the U.S. National Oceanic and Atmospheric Administration (NOAA) National Geophysical Data Center (NGDC) in Boulder, Colorado. Elvidge and his colleagues improved the intercalibration between the two time periods, correcting for differences in sensors on different DMSP satellites and slight changes in their behavior over time while in orbit. This enables more detailed and precise analysis of urban development patterns over time, especially in areas of the world where conventional sources of information about population and urbanization are not available.
This blog is part of the Map of the Month blog series produced by the Center for International Earth Science Information Network (CIESIN). The map was made by senior research staff assistant Erin Doxsey-Whitfield. Text was developed by deputy director Marc Levy, senior staff associate Sandra Baptista, and geographic information specialist Linda Pistolesi.