Patent ID: 8779897

Claim:
A method of producing location information relating to articles provided with tag devices each tag device comprising a memory unit for storing unique identification data associated with the respective tag device, a first communication unit for communicating with other tag devices when in the proximity of the other tag devices, and a clock unit, the method comprising the steps of: each tag device during an encounter with another tag device, exchanging identification data and storing the time and date of the exchange together with the identification data as encounter data, each tag device, when in the proximity of an information collection device comprising a memory and a second communication unit, communicating the stored encounter data to the information collection device, the information collection device communicating the encounter data received from tag devices to a processing device, and the processing device deriving location information from the encounter data, wherein each tag device, during each encounter with another tag device, also exchanges and stores encounter data relating to previous encounters, such that the encounter data which each tag device communicates to the information collection may relate to encounters of a plurality of tag devices.