5.2 Exploiting Spatial Context So far, we have only mentioned the geographic position, given as x-y-z co-ordinates, which is a precondition for location-awareness. In order to offer location based services (LBS), the logical or topological position of a user is equally important. This is, for instance, the position in terms of addresses, town name or town area. This is also the topological relation to other geographical objects, like "on Trafalgar Square" or "in front of St. Paul's Cathedral". We call this the spatial context of the user. Most services are not based on co-ordinates but on addresses and names of areas (e.g. country, town, or area in a town). The translation of a coordinate to an address or area, which can be used to find a service or facility by a kind of yellow pages, has to be done by a separate geo-coding service. It is also necessary to have the inverse translation, i.e. from area or address to coordinates, because this enables more helpful features. For instance, the distance between a destination and the current position of the user can thus be calculated.
• 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.