These maps show you every tree in your city

"You can use either aerial imagery or satellite imagery to do basically the same task, but a lot faster," says Aidan Swope, a Caltech undergrad who created the algorithm as an intern at the tech startup Descartes Labs. Because taking a census by hand takes months or years, some trees are inevitably cut down before it's complete, so the final map won't be completely accurate. And these censuses typically also only include street trees, not trees in parks or on private property, while the algorithm includes everything. The tool uses a convolutional neural network, similar to those used for facial recognition. While it's not hard for a machine to find green areas in an aerial image, Swope also trained the model with lidar data, a type of remote sensing data that shows height, making it possible to distinguish trees from grass or other plants.