User-Generated street Maps

T he process of mapping the Earth accurately was, until recently, the preserve of highly skilled, wellequipped, and organized individuals and groups. For many years, it was usually the role of surveyors, cartographers, and geographers to map the world and transcribe it on paper or, since the 1960s, into the computer. Lewis and Clark’s expedition to map North America’s West, and Lambton and Everest’s Great Arc expedition to measure India, are just two famous episodes in the history of maps and map making. Each country has an established national mapping agency charged with keeping the national maps accurate and current (for example, the US Geological Survey and the UK Ordnance Survey). Less than a decade ago, it was common to assume that a person needed a university-level degree to be able to measure the Earth and transcribe the information on paper or into the computer as well as expensive equipment and infrastructure to support his or her work. This, however, has changed dramatically over the past decade. On 1 May 2000, US President Bill Clinton announced the removal of selective availability of the GPS signal1 and, by so doing, provided much improved accuracy for simple, low-cost GPS receivers. In practical terms, this made it possible to acquire the receiver’s position with an accuracy of 6 to 10 meters in normal conditions, in contrast to roughly 100 meters before the “switch off.” Attempts to develop locationbased services predated this announcement2 and were based on information from mobile phone masts or other beacons. However, these methods hadn’t gained much market share owing to their technical complexity and inability to provide a universal coverage. In contrast, GPS enabled the development of cheap receivers with good positional accuracy, and, by mid2001, it was possible to purchase a receiver unit for roughly US$100.3 These receivers helped more people than ever before collect information about different locations and upload it to their computers. However, until 2002, when an interchange standard (GPS eXchange format or GPX) was published, manipulating and sharing this information was a complicated task that required computing and data manipulation knowledge. Fortunately, most GPS receiver developers rapidly adopted the GPX standard, and, by 2004, it had become commonplace (www.topografix.com/gpx.asp). The wide availability of high-quality location information has enabled mass-market mapping based on affordable GPS receivers, home computers, and the Internet. Although a range of projects based on user-generated mapping has emerged, OpenStreetMap (OSM) is probably the most extensive and effective project currently under development. In this article, we review the project and provide an overview for the techniques and methodologies used within it.