ARMED with a Global Positioning System receiver and a pair of itchy feet, Jo Walsh walks a different route around town each week. She is slowly but steadily building a digital map of her neighbourhood in Bristol, UK. In doing so, Walsh is reinventing the pioneering spirit, for she is one of hundreds of people using cheap, off-the-shelf satellite tracking equipment to make their own maps.

The principle is simple. Set your GPS receiver to record longitude and latitude at frequent intervals during a walk, bike ride or car trip, then download the information to a computer and watch as it traces out your journey on screen. And by combining data from various trips, you'll get a rough but usable digital map of the world you live in. "You just need a GPS receiver and data cable and then anyone can do it," says Walsh's husband and collaborator Schuyler Erle. ...