A method of delivering location-based information comprises receiving a request including a location indicator from a packet-based network and translating the location indicator into a probabilistic representation of one or more possible locations of a mobile user based on historical data collected from mobile devices. By generating the probabilistic user location representation, the method can be used to deal with the inherent uncertainty in user location data, particularly in a mobile world. In some embodiments, this is done by using validation, context analysis, creation of consistent groups, and probability allocation procedures. Location-based document retrieval can be enabled using the probabilistic user location representation. In certain embodiments, IP regions for respective IP addresses are derived from ad requests made over a certain period of time, and the IP regions are used to translate location indicators of subsequent ad requests into probabilistic representations of possible locations of mobile users.