1. A method, comprising: determining signal source probabilities for a plurality of sub-areas of a geographic area covered by a plurality of signal sources, wherein, for each one of the plurality of sub-areas: the signal source probabilities are determined based on probable field strengths of the plurality of signal sources for the one of the plurality of sub-areas; and the signal source probabilities indicate the probability that a mobile device, located in the one of the plurality of sub-areas, is operable to detect a particular signal source for communicating.