"Nighttime imagery is used as a tool to look at stable populations, which is the opposite of what we used it for," Bharti said. "Setting up this latest project was very labor-intensive. The idea of applying nighttime-lights data in this way is somewhat unconventional, so there was no previous research for us to work from."
Follow the lights, follow the crowd
The work stems from a longtime effort in Grenfell's lab to understand seasonal measles epidemics in Niger. In 2010, Bharti published a paper with Djibo, Grenfell, Grais and lead author Ferrari, as well as Penn State entomology professor Ottar Bjornstad, reporting that measles epidemics in Niger only occur during the dry season and that an outbreak's severity is related to an area's population.
Thus, the researchers concluded, these events are likely the result of population shifts, rather than environmental factors such as rainfall. But without an accurate method for measuring population movement and changes in density, they could not test their hypothesis, Bharti said.
The project reported in Science is intended to provide such a method. The researchers selected nighttime images clear of excess light pollution and obscuring weather from several hundred photos captured between 2000 and 2004 by the Defense Meteorological Satellite Program's Operational Linescan System, operated by the U.S. Department of Defense. Those images were compared to records from Niger's Ministry of Health of weekly measles outbreaks during the same years in the country's three largest cities: Maradi, Zinder and the capital, Niamey.
Seasonal brightness for all three cities changed similarly, the researchers report. Brightness was below average for each city during the agriculturally busy rainy season, then rose to above average as people packed urban areas during the dry season. Measles transmission rates followed the same pattern low in the r
|Contact: Morgan Kelly|