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April 29, 2007

Collecting Imagery for Google Earth

Mark Aubin, co-founder of Keyhole (the product known these days as Google Earth), provides some details about the process of collecting satellite imagery. Google has many data providers and the imagery is updated at least every three years.
Most people are surprised to learn that we have more than one source for our imagery. We collect it via airplane and satellite, but also just about any way you can imagine getting a camera above the Earth's surface: hot air balloons, model airplanes – even kites. The traditional aerial survey involves mounting a special gyroscopic, stabilized camera in the belly of an airplane and flying it at an elevation of between 15,000 feet and 30,000 feet, depending on the resolution of imagery you're interested in. As the plane takes a predefined route over the desired area, it forms a series of parallel lines with about 40 percent overlap between lines and 60 percent overlap in the direction of flight. This overlap of images is what provides us with enough detail to remove distortions caused by the varying shape of the Earth's surface.

The next step is processing the imagery. We scan the film using scanners capable of over 1800 DPI (dots per inch) or 14 microns. Then we take the digital imagery through a series of stages such as color balancing and warping to produce the final mosaic for the entire area.

We update the imagery as quickly as we can collect and process it, then add layers of information – things like country and state borders and the names of roads, schools, and parks - to make it more useful. This information comes from multiple sources: commercial providers, local government agencies, public domain collections, private individuals, national and even international governments. Right now, Google Earth has hundreds of terabytes of geographic data, and it's growing larger every day.


  1. Google Earth is a perfect example of why I think the rules of the world are changing. People who advocate the idea of history eternally repeating should consider Google Earth : Never in the history of man has a technology existed that permitted literally anyone access to information this real, and it only continues to improve.

  2. It's a shame so much of the stuff outside the US is so out of date, or of very low resolution.

  3. would like to know the date when the googoo bird is overhead I want to mow my email into my lawn and paint my email on my roof

  4. Ugh, I live in Northern Virginia, near Washington, DC, but my whole county is still a green blob. You can only zoom very slightly, which does nothing but add some pixelation to the green blob.

    I know my neighborhood is relatively new (~10 years), but I can't even tell whether it's farmland or a community, from Google's satellite imagery.

    As soon as you pan over to a neighboring county, though, you can zoom in and almost identify cars in driveways.


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