In a computer system, a pedestrian dataset that indicates position fixes for several portable devices is received, such that each portable device corresponds to a respective pedestrian. Raw heat scores for several geographic units of equal size are generated based on the pedestrian dataset, each raw heat score being indicative of a number of position fixes in the corresponding geographic unit. A selection of a geographic area that contains some of the geographic units is received, and normalized heat scores for these geographic units are generated based on at least some of the generated raw heat scores.