Next, Landsat's operator, the U.S. Geological Survey, altered its policies to make all of the data from Landsat 7 and previous Landsat satellites free. Previously, Hansen said, researchers had to buy the data piecemeal. It would have cost millions to purchase the data for the entire globe.
"We never had the data we needed," he said. "We had the data we could afford."
Finally, with access to all the satellite data came the need for major computing power to process it. Hansen and his colleagues teamed up with Google to make it happen. On a single computer, processing the data archive would have taken 15 years, Hansen said. With Google's cloud computing, it took mere days.
The fine scale of the map allows researchers to zoom in close enough to see logging roads, river meanders and even tornado tracks, Hansen said.
"There are a ton of stories here," he said. Some of the information that comes from forest maps is entirely unexpected, he added. One researcher took another of Hansen's maps and found that tree cover correlates with human health, because forest dwellers eat a more diverse diet than people in other environments do.