1. A method of predicting geographic population density, comprising: defining a Markov Random Field (MRF) model representing a plurality of state variables as nodes in a lattice, each node in the lattice corresponding to a geographic area such that neighboring nodes correspond to neighboring geographic areas, and each state variable representing an estimate of a number of mobile devices being used in a corresponding geographic area based on historical data describing mobile devices being used in the geographic area; applying, by a computer processor, an observation of a number of mobile devices currently within the geographic area to the MRF model to produce an estimate of a number of mobile devices currently being used within the geographic area; and storing the estimated number of mobile devices currently being used within the geographic area in a computer-readable storage medium.