1. A method comprising: measuring scattering properties of a first target comprising a first structure; constructing a model of the first structure using the measured scattering properties, the model comprising a first model structure corresponding to the first structure; modifying the model based on asymmetry of the first model structure; calculating an asymmetry-induced overlay error between the first model structure and a second model structure, the first and second model structures being overlaid with respect to each other in the modified model; and determining an overlay error in a second target using the calculated asymmetry-induced overlay error.