A Tree Grows in... Pasadena: How many--and what kind of--trees are in the urban landscape? An algorithm developed at Caltech may provide the answer.

• Author: Svitil, Kathy
• Position: THE ENVIRONMENT - California Institute of Technology, Pasadena, California

A METHOD that uses data from satellite and street-level images--such as the ones that you can see in Google maps--to create an inventory of street trees that cities may use to better manage urban forests has been developed by engineers from the California Institute of Technology.

"Cities have been surveying their tree populations for decades, but the process is very labor intensive. It usually involves hiring arborists to go out with GPS units to mark the location of each individual tree and identify its species," says Pietro Perona, professor of electrical engineering and computation and neural systems. "For this reason, tree surveys are usually only done every 20 to 30 years, and a lot can change in that time."

Perona and his team are not expert arborists. Rather, they are leaders in the field of computer vision. They specialize in creating visual recognition algorithms--computer programs capable of "learning" to recognize objects in images--that can see and understand images much like a human would. These algorithms, by replicating the abilities of specialists, sometimes even can understand images better than the average person. As part of a project called "Visipedia," a collaboration with Serge Belongie of the Joan & Irwin Jacobs Technion-Cornell Institute and Cornell University and the Cornell Lab of Ornithology, the engineers have developed algorithms that can recognize the species of a North American bird from a single picture.

The team eventually hopes to develop Visipedia's capabilities until it accurately can recognize nearly all living things, but they were inspired to turn their attention toward trees when Perona noticed the effects of the years-long California drought on the trees near the Caltech campus in Pasadena.

"I happened to notice that many people in Pasadena were putting drought-resistant plants in their yards to save water but, when they took out the lawns and stopped watering, many trees started dying, and that seemed like a shame. I realized that computer vision might be able to help. By analyzing automatically satellite and street-level images that are routinely collected, maybe we could carry out an inventory of all the trees and we could see over time how Pasadena is changing, whether the trees that are dying are just a few birch trees, which are not native to California and require frequent watering, or whether it's truly a massive change."

To begin their survey of the Pasadena urban tree population, the team...