The world is a surprisingly cloudy place when you step back and view it from space, sharing the vantage point of satellites. But while pictures like the one below have a beauty in their own right, they're particularly problematic for map-makers more interested in the obscured details of the earth's surface than its atmosphere:
That picture is a composite of images taken just last week by a pair of NASA satellites that each circle the globe twice a day. Data from such sources are used to create the satellite base layer for many maps, and so, as you can imagine, clouds easily get in the way. Whole regions of the world around the earth's midriff – in the "intertropical convergence zone" where northern and southern hemisphere weather patterns meet and mix – may be visible for only a few clear weeks in a year.
"These mountains along the equator are a huge challenge," says Charlie Loyd, an imagery specialist at MapBox. He rattles off an entire atlas of inaccessible sites: parts of the Andes Mountains in South America, Mount Cameroon in West Africa, the mountains of New Guinea and Borneo, all of which are perpetually shrouded in clouds. "Anything we can do to get rid of them," Loyd says, "gives us maps that are more useful to people and more pleasing to look at."
Until now, though, methods for doing that – erasing the atmosphere – have been imperfect. Sometimes satellite maps will awkwardly stitch together side-by-side images taken months apart in different seasons (Google Earth is full of such seams). Loyd and MapBox, though, appear to have finally come up with an imperceptible method of removing clouds.
Instead of stitching together views of the world like a patchwork quilt, Loyd and colleagues have mined two years' worth of images from those NASA satellites mentioned above for the single clearest pixels. Those satellites capture any given location on the earth twice a day, creating around 730 images of a single place over the course of a year (or more than 1,400 in a two-year time period). Consider, for instance, those hundreds of images from a single lake somewhere.
"We take all of the pixels that represent that lake over a long time span," Loyd explains, "and we look at the roughly one-tenth of those pixels that are the clearest, and we average all of those together – all of their colors – and we smooth out over the very clearest days."
Here is the resulting image from a part of South America spanning Ecuador, Peru and Brazil:
The city of Guayaquil, with more than 2 million people, sits on the estuary of the Guayas River in the northwest of the image. It's development is visible in the absence of greenery. Immediately to the right of it lies the main ridgeline of the northern Andes, with the highest mountains on the eastern side of the range still peaked in snow.
Now here is what this same part of the world looked like from a satellite view taken earlier this week:
That latter image more or less shows a typical day in the Andes.
To create this clearer picture of the world, MapBox had to process 339,000 individual images from NASA, a terabyte of data captured between Jan. 1, 2011 and Dec. 31 of 2012, downloaded over the course of several days.
"After three days," says MapBox data analyst Chris Herwig, "we had the world for two years."
Later this month, MapBox plans to incorporate the new imagery into the relaunch of its MapBox Satellite base layer, upon which other people will be able to build maps of all kinds. The resulting view of the world will play with time, rendering every spot on the planet as if on a cloudless day. The effect will look something like perpetual summer, in both hemispheres simultaneously.
"It was really amazing to see the first [images] come out," Loyd says, "to put them on a map and see just how intense and vivid they were."
Here are a few more early previews of the project, all courtesy of MapBox (the top image shows Baffin Island in the Canadian Arctic).