A metrology system is provided comprising a diffraction grating mounted on the object, and an interferometer head operable to direct at least one measurement light beam towards the grating at a non-zero angle of incidence. With a single first separation in the second direction between the object and the interferometer head, respective displacement measurements are made at known displacements of the object. From the displacement measurements are generated respective original calibration values, each pertaining to a respective one of the known displacements. With a second separation in the second direction between the object and the interferometer head, a displacement of the object is measured. The measured displacement is corrected using an offset calibration value derived from at least one of the original calibration values and pertaining to at least one offset displacement, offset from the measured displacement.