A method of aligning a pair of complementary diffraction patterns having a first complementary diffraction pattern and a second complementary diffraction pattern, the pair of complementary diffraction patterns obtained from performance of a metrology process on a structure formed by a lithographic process. The method includes performing at least a fine alignment stage to align the pair of complementary diffraction patterns. The alignment stage includes: interpolating measured values of the first complementary diffraction pattern over at least a portion of a detector area; and minimizing a residual between measured values in the second complementary diffraction pattern and corresponding interpolated values from the interpolation of the first complementary diffraction pattern, by one or both of translation and rotation of the second complementary diffraction pattern.