Using night lights to measure economic growth

(Image: Xi Chen)

When you're rich, you turn on the lights.
It might seem a crude observation at face value, but researchers have discovered that a brightly lit country at night can help indicate how wealthy it really is.
Using familiar birds-eye satellite images of cities at night, researchers from Yale and Quinnipiac Universities in the US analysed US Department of Defense imagery from 1992 until 2008.
The team divided the world up into cells and measured each cell's "luminosity" - how bright each cell was. Even this process was mired in technical difficulties as obstacles such as automatic correction for glare, cloud cover and distortions caused by water vapour have to be removed. A figure was attributed to the brightness of each country's grid cells and these figures were combined and then aggregated over a year to give an annual luminosity value.
Official GDP figures from the World Bank were matched with each year to see if there was any correlation. A series of statistical calculations were then run to see whether an increase in light over time matched an increase in economic output - where it was known.
The results were far from conclusive. What the team found was that the figures were close but not quite precise enough to be useful for large, wealthy countries such as the US or Australia. This was because the margin of error in analysing the luminosity value was far larger than that in the official GDP figures.
However they found that when they turned their gaze on poorer and developing nations the match between actual economic output and night time luminosity provided rather more insight. In some countries like Cambodia, Somalia or North Korea - where there is very little economic data and there hadn't been a recent population census - analysis of the cells helped trace how well a nation was faring.
And in some areas, particularly the Democratic Republic of Congo, the luminosity counts helped track the country's economic health over time, where it had been difficult to do so before because of instability or violence.
The research is not the first to assume that lights at night are analogous with prosperity but is the first to look at compare actual economic figures with luminosity in this way.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1017031108
Niall Firth @'New Scientist'