Researchers at Carnegie Mellon University have devised the first computerised method that can analyse a single photograph and determine where in the world the image was likely to have been taken.
It's a feat made possible by searching through millions of GPS-tagged images in the Flickr online photo collection.
The IM2GPS algorithm developed by computer science graduate student James Hays and Alexei A. Efros, assistant professor of computer science and robotics, does not attempt to scan a photo for location clues, such as types of clothing, the language on street signs, or specific types of vegetation, as a person might do.
Rather, it analyses the composition of the photo, notes how textures and colours are distributed and records the number and orientation of lines in the photo. It then searches Flickr for photos that are similar in appearance.
'We're not asking the computer to tell us what is depicted in the photo but to find other photos that look like it,' Efros said. 'It was surprising to us how effective this approach proved to be. Who would have guessed that similarity in overall image appearance would correlate to geographic proximity so well?'
Hays and Efros found they could accurately geolocate the images within 200km for 16 per cent of more than 200 photos in their test set - up to 30 times better than chance. And even if their algorithm failed to identify the specific location, they often found that it could narrow the possibilities, such as by identifying the locale as a beach or a desert.
'It seems there's not as much ambiguity in the visual world as you might guess,' said Hays. 'Estimating geographic information from images is a difficult, but very much a doable, computer vision problem.'
Identifying the locale of a photo could enhance image search techniques, making them less dependent on captions or associated text. A computer system for geolocating photos could be useful in finding family photos from a specific trip and in some forensic applications. Determining the location of photos also makes it possible to combine them with geographic data bases related to climate, population density, vegetation, topography and land use.
Hays said many online photos have some sort of geographic label, but these human descriptions can often be incorrect, or overly broad, such as a photo of the Grand Canyon labelled 'US'.
The growing number of online photos that have GPS tags, by contrast, are unambiguous regarding their location, even though many are photos of rooms, people or events such as birthday parties that are useless for geolocation tasks. By using photos with both geographic keywords and GPS coordinates, Hays and Efros were able to find more than six million photos that were useful and accurately geolocated.
The IM2GPS algorithm readily located photographs of such landmarks as the Cathedral of Notre Dame in Paris. More surprisingly, it was able to recognise that a narrow street in Barcelona was typical of Mediterranean villages, rather than an American alleyway.
But some odd matches also occurred. The architecturally unique Sydney Opera House seemed to the computer to be similar to a hotel in Mississippi as well as a bridge in London. A shot of the Eiffel Tower at dusk was matched to other Eiffel Tower shots, but also to San Francisco's Coit Tower and New York's Statue of Liberty, both shot at dusk.
One reason for this confusion, Hays explained, is that the algorithm is not designed to recognise specific objects so much as it is to recognise geographic areas. For instance, an image of Utah's Monument Valley caused the IM2GPS algorithm to successfully retrieve a number of other images from Monument Valley and the American Southwest, rather than images of a specific rock formation.
For more information, see the IM2GPS project Web site: http://graphics.cs.cmu.edu/projects/im2gps/