A method for estimating a parameter across a region on a substrate, the region being divided into a plurality of sub-regions, the method including: obtaining values of the parameter for at least two sub-regions out of the plurality of sub-regions; and estimating the parameter for a position on the region by evaluation of a function having said values of the parameter as input values, wherein the function: a) has piecewise defined base functions, wherein a single base function is defined across a sub-region; and b) is continuous between one or more adjacent sub-regions of the at least two sub-regions within the region.