Abstract: A method and apparatus for processing a distribution of a population (4) distributed over a region of interest (2), the region (2) being divided into cells, the method comprising: measuring, at two different points in time, values of a measure of population intensity in each cell; for a cell, determining a vector of values, each value in the vector being a function of the intensity values measured in that cell and the population intensity values measured in a different cell; determining a matrix using the determined vector(s) and the population intensity values measured at one of the different points in time; and predicting, using the matrix and the value of the population intensity measured in a cell at either of the different points in time, a value for the measure of population intensity in that cell for a point in time different to the two different points in time.