Systems and method to measure a population size of mobile devices that have visited a region during a time period, by: converting location data of mobile devices into visitation data using a grid reference system, where for each mobile device and each of a plurality of time intervals in the time period, the visitation data identifies whether or not the mobile device has visited the region in the time interval; applying the visitation data as a capture history in an imaginary capture-recapture exercise, where whether or not a device has visited the region during a time interval represents whether the device is captured in the exercise in the respective capture occasion corresponding to the time interval; and estimating the population size based on an estimate of a population size of mobile devices that are not captured in the exercise from a capture-recapture statistical modeling of the capture history.