A metrology system includes a radiation source configured to generate radiation, an optical element configured to direct the radiation toward a grating structure comprising a non-constant pitch, and a detector configured to receive radiation scattered by the grating structure and generate a measurement based on the received radiation. The metrology system is configured to generate a set of measurements corresponding to a set of locations on the grating structure along a direction of the non-constant pitch and determine a parameter of a lithographic process or a correction for the metrology system based on the set of measurements.