Picture (Im)perfect

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A picture may be worth a thousand words. But, as it turns out, it takes almost 100 million pictures to make a map.

The inventive engineers at Flickr--a popular Web site that allows users to upload and share photos online--have discovered a way to harness the data provided by their millions of users to create a constantly changing picture of the world itself.

When a user uploads a photo onto Flickr, he or she can pinpoint, or "geotag," the location where that photo was taken on an interactive map. GPs-enabled camera phones, such as Apple's iPhone, can do this automatically. Flickr then uses these coordinates to create a "Where on Earth" ID for the photo that includes the neighborhood or town where it was taken, right up to the continent, a process known as reverse-geocoding. The actual content of the photo itself is irrelevant; it's simply being used for its geographical data.

With 90 million geotagged photos and counting, the company's development team realized that these locations could be plotted on a map to create an outline.

Not all locations are equally easy to plot. "Within the first few weeks [of geotagging] there were probably enough photos to map San Francisco," says Flickr's Dan Catt, the senior engineer spearheading the project. …