1. A computer-implemented method, comprising: comparing, using one or more computer-processors, a scanning interferometry signal obtained for a location of a test object to each of multiple model signals corresponding to different model parameters for modeling the test object; wherein for each model signal the comparing comprises calculating a correlation function between the scanning interferometry signal and the model signal to identify a surface-height offset between the scanning interferometry signal and the model signal and, based on the identified surface-height offset, calculating a height-offset compensated merit value indicative of a similarity between the scanning interferometry signal and the model signal for a common surface height, wherein calculating the height-offset compensated merit value comprises compensating a frequency domain representation of the scanning interferometry signal or a frequency domain representation of the model signal with a linear phase term having a slope corresponding to the identified surface-height offset and quantifying the similarity between the scanning interferometry signal and the model signal following the phase compensation; and based on the respective merit values for the different model signals, determining a test object parameter at the location of the test object.