1. A method comprising: determining a first measurement model of a target structure located on a semiconductor wafer, the first measurement model including a first set of parameters; determining a cross-wafer model that characterizes a process variation of at least one parameter of the first set of parameters at any location on the semiconductor wafer; receiving a first amount of measurement data associated with measurements of the target structure by a metrology tool; determining a second set of parameter values characterizing the target structure based on a fitting of the first amount of measurement data to a second measurement model of the target structure that is constrained by the cross-wafer model; and storing the second set of parameter values in a memory.