1. A method comprising: measuring scattering properties of a first target comprising a first structure having an asymmetric distortion; constructing a model of the first structure using the measured scattering properties, the model being a parameterized model that reconstructs the shape of first structure and the asymmetric distortion; modifying the model based on the asymmetric distortion of the reconstructed first structure; calculating an asymmetry-induced overlay error between the reconstructed first structure and a second model structure, the reconstructed first structure and the second model structure 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.