Google Earth - How They Do It!

A look at how Google Earth puts together images from all over the world.

When you launch GoogleEarth on your computer, you start in outer space and you miraculously zoom in. You might see two unsuspecting guys walking across the Blue Wonder Bridge way off in Dresden, Germany.

But the folks at Google Earth remind us that you're not zooming in on just one picture. You're actually going through a succession - seamlessly - of closer and closer shots, making the transition from a NASA shuttle shot to a satellite shot to a photograph made from an airplane. So that's how they get such good close-up resolution.

Google doesn't shoot its own images. There are a handful of companies that do that. As you zoom in, down at the bottom of your screen you'll see names like AeroWest, DigitalGlobe, GeoContent, Cnes/Spot Image, NASA and Terra Metrics. Those are the various suppliers of images. But GoogleEarth has designed software that knits it all together so it feels like we're zooming in.

The primary source of GoogleEarth images is, DigitalGlobe. They told Pop Photo how the system works.

At the heart of it all is a satellite in a low orbit, going from one pole to the other. It's different from geostationary communications satellites because it's at 20,000 miles and always above the equator. You can't get a decent shot from that height. And the angle is limiting. For example, you'd always get a shot of the Statue of Liberty's front profile since she faces southeast. You'd never get a straight-down shot. The only alternative is to use a camera inside a satellite that's swooping from pole to pole at a much lower altitude, typically about 300 miles.

Chuck Herring at DigitalGlobe says they have two satellites to choose from, with a third launching next year. But the one they use the most, named QuickBird, makes a polar orbit every 90 minutes.

The camera is unique, not like a DSLR that exposes all the pixels at once. There's no shutter. The camera is designed more along the lines of a flatbed scanner hooked up to a very, very long telephoto lens. As the satellite floats along, the image of the earth in effect moves beneath it. The imaging bar, called a "push broom sensor," covers an area on the ground that's 16.8 km wide (nearly 10 miles). It focuses the image onto a CCD array that is 13 inches wide and contains nearly 30,000 pixels. And the resolution is astounding. Herring says each pixel covers an area on the ground about the size of home plate, which allows you to clearly see a car.

The lens itself is gigantic. Imagine a telephoto lens 30 feet long! That's 8.8 meters. It has a fixed aperture of f/14.7. Kind of slow but they always shoot in the daytime. And of course, only on sunny days! (Who needs close-ups of cloud tops?) Typically they shoot with the sun at an angle to give better definition. They can even make 3-D shots by combining images shot at an angle. The camera can point up to 20 degrees from vertical.

So how do you fit a 30-foot lens inside a satellite? The engineers at DigitalGlobe call it "folded optics": lots of mirrors bouncing the image back and forth. The camera produces an image that's generally rectangular - 16 km wide and 250 km long. That's roughly 10 miles by 155 miles. But remember, that's the size of the image, not the size of the sensor. The sensor is basically just a slit. Theoretically they could shoot a continuous strip around the entire earth. But there's not enough storage space on board for that much data, so they cut it off at 155 miles.

But what about that detailed shot of two guys walking across the bridge in Dresden, Germany? That's where the wizardry of software comes in. Numerous suppliers - both public and private - have images shot from airplanes. The trick, of course, is to piece them together. GPS data makes it all possible. Plus, the people at GoogleEarth have their own proprietary software that knits the images together. They wouldn't go into detail but said it's something like the software that you and I might use to knit together a panoramic image.

GoogleEarth has an addition that's stirring up some controversy: "street level" mapping. Basically it's a rig on top of a vehicle containing eight cameras, covering a full circle as it slowly drives along, snapping digital images. If you happen to be in one of those pictures and don't want to be, ask Google and they'll blur out your face. If you were in the witness protection program, you might be a bit miffed if everybody around the world saw you.

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