It took nearly a decade to build the Colosseum, and almost a century to construct St. Peter's Basilica. But now with the help of loads of tourist photos and some fancy new software, these landmarks and the rest of Rome can be digitized in about a day.

A new computer algorithm developed at the University of Washington uses hundreds of thousands of tourist photos to automatically reconstruct an entire city in a matter of hours. The result is a 3-D view of landmarks, created by stitching together photos from various angles.

The digital Rome was built from 150,000 tourist photos tagged with the word "Rome" or "Roma" that were downloaded from the popular photo-sharing Web site, Flickr. Story continues below.

{{ embed="20090915" }}

{{ embed="20090915" }}

Computers analyzed each image and in 21 hours combined them to create a 3-D digital model. With this model a viewer can fly around Rome's landmarks, from the Trevi Fountain to the Pantheon to the inside of the Sistine Chapel.

"How to match these massive collections of images to each other was a challenge," said Sameer Agarwal, a UW acting assistant professor of computer science and engineering and lead author of a paper being presented in October at the International Conference on Computer Vision in Kyoto, Japan. Until now, he said, "even if we had all the hardware we could get our hands on and then some, a reconstruction using this many photos would take forever."

Earlier versions of the UW photo-stitching technology are known as Photo Tourism. That technology was licensed in 2006 to Microsoft, which now offers it as a free tool called Photosynth.

"With Photosynth and Photo Tourism, we basically reconstruct individual landmarks. Here we're trying to reconstruct entire cities," said co-author Noah Snavely, who developed Photo Tourism as his UW doctoral work and is now an assistant professor at Cornell University.