"BERKELEY Square is turning at this moment into a sort of well, sandwiched in between monster blocks of windows scraping as much of the sky as the by-laws will allow."

So said the poet John Betjeman in a January 1939 radio broadcast in which he lamented the growing number of high-rise buildings in London.

Now we can watch for ourselves the way great cities have grown upwards through time, using software that creates a virtual historical tour. Called 4D Cities, the software can automatically sort a collection of historical city snapshots into date order. It then constructs an animated 3D model that shows how the city has changed over the years.

The idea is to give architects, historians, town planners, environmentalists and the plain curious a new way to look at cities, says Frank Dellaert at the Georgia Institute of Technology in Atlanta, who built the system with his colleague Grant Schindler and Sing Bing Kang of Microsoft's research lab in Redmond, Washington.

"The system shows the city changing over time as old buildings are demolished and new ones are constructed," says Schindler. "You'll be able to see what the view out of your window looked like 100 years ago."

To create a model of Atlanta, the researchers scanned in numerous historical photos of the city that had been snapped from similar vantage points. The software is designed to identify the 3D structures within the image and break them down into a series of points. It then compares the view in each one to work out why some of these points are visible in some of the images but not others. Was the building simply out of shot? Or was the view of one building blocked by another? "If we can rule out those two possibilities, then we know that the reason we don't see a building is because it didn't exist when the image was taken. Either it was not yet built or it had already been demolished," says Schindler.

The software continually rearranges the order of the images taken from each vantage point until the visibility patterns of all the buildings are consistent, Schindler says. "Buildings are not allowed to vanish and then reappear without explanation."

The result is that the images appear in time order, allowing the researchers to construct and animate a 3D graphic of the city through which users can travel backwards or forwards in time (www.cc.gatech.edu/~phlosoft).

The researchers plan to extend the system to create models of other cities, and to improve the software's ability to recognise whether different photos are showing exactly the same scene. This can be difficult as some cityscapes change so profoundly. Once they do this, they will be able to tackle much larger sets of time-sequenced images, says Schindler.