A Better Way to Map Humanity's Spread Around the Earth

As the human population grows, so does its footprint. To map these changes, researchers often turn to satellite imagery, because government-collected data can be infrequent and outdated. In particular, nighttime light images can offer a wealth of information about human activity. In fact, as CityLab’s Richard Florida has written, more than 3,000 studies since 2000 have used nighttime lights as a proxy for all sorts economic activities.

But nighttime maps aren’t perfect. “If you need to figure out how large a city is and where the boundary of a city ends, lights will spread, and a city will look too large relative to its actual size,” says Amit Khandelwal, director of the Chazen Institute for Global Business at Columbia Business School. And there’s another problem: Satellite sensors have a saturation point that limits their ability to distinguish between different levels of light. That means an extremely bright place like Midtown Manhattan may appear to be equally bright as other parts of New York, even though it has more activity.