1. A method for determining a parameter of a target pattern, the method comprising: determining, with a processing device, a first plurality of calibration spectra from a reference pattern, each of the spectra are determined using a different known value of at least one structure parameter of the reference pattern, wherein the first plurality of calibration spectra is independent of parameters of an apparatus used to produce the reference pattern; storing a representation of each of the first plurality of calibration spectra in a first library in a memory; determining, with the processing device, a second plurality of calibration spectra for a target pattern using at least one of the stored spectra in the first library and parameters of a measuring apparatus configured to measure the target spectrum; measuring, with the processing device, a spectrum by directing a beam of radiation onto the target pattern; comparing, with the processing device, the measured spectrum and the second plurality of calibration spectra; and using, with the processing device, the comparison to derive a value for the parameter of the target pattern.