5.1 User Position in Physical Space. In some modern PDA devices, the sensors for determining the location by means of GPS are already built in or can easily be attached. CRUMPET will use them to enable the features mentioned above. Positioning by GPS states the client's geographical position as x-y-z co-ordinates. So, if a user asks for information relevant to his current position, the system can automatically complete this simple request using the parameter “current position”. In order to make sensible use of wireless connection, the client device needs some intelligence to interpret the current position and send only the relevant position coordinates to the middle-agents residing on the server. The question, which position or rather which change in a user's position is relevant is not easily answered. The assessment of "relevance" of a movement has to take into account several factors.
• The general precision of GPS (if the precision is 30 m, for instance, a move of 10 m, however relevant, cannot be reliably measured). • The last n positions (if the precision is 30 m, and several small changes accumulate to a movement of more than 30m in one co-ordinate, this is relevant). • The type of environment (e.g. a small movement that is relevant on a building site might be less relevant within an urban environment, and irrelevant on the countryside). • The current velocity (e.g. if the user moves slowly, the intervals of observing the position and assessing its relevance could be stretched). For positioning there are also other techniques available, for instance infrared sensors, Bluetooth, or GSM/UMTS. For certain applications it might be useful to complement GPS for outdoor positioning with infrared positioning. This is very precise and can also be used to directly identify a location without reference to a GIS. It requires, however, the installation of sensors on a site. This can be feasible and efficient in a restricted area, such as a campus, an amusement park, or within a museum. In addition to positioning, the orientation of the user can be of interest. An electronic compass in the client device can help to interpret the location of the user, and offer more specific information. A privacy concern is that users may not want their whereabouts to be constantly observed by a system. Therefore it must be possible that the user can switch off the positioning. Of course, the system support in that case is limited, and the user has to explicitly input the positioning parameters required for location-based services.