sensors. Obviously, this is not suitable for immediate use or mass market applications, but it serves as starting point from which other work could be derived. During a research fellowship at the University of Georgia in 2003, Kit Hughes developed a system in which users with WiFi-enabled mobile devices could mark up buildings in downtown Athens, Georgia, with their own virtual graffiti via a process known as tagging. In this system, the buildings are selected on a map, and the graffiti is stored in a database that can be accessed from other mobile devices and the project’s website. A location-based messaging system for leaving virtual post-it notes on physical objects was developed at the National University of Singapore. The system uses mobile devices as AR interfaces to view virtual messages associated with fiducial markers on physical objects. In a project from Lancaster University, mobile phones are used as digital mediums, using RFID tags to identify markable objects. The RFID tags can hold the identities of the last five people to leave graffiti. The graffiti itself is stored on a server. When another user comes within range of an RFID-tagged object, the associated graffiti is downloaded onto their mobile device. See also WallaMe Locative media Location-based game Global-positioning system Urban exploring References Virtual reality Augmented reality Graffiti and unauthorised signage