1. A method, comprising: comparing 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; and based on the respective merit values for the different model signals, determining a test object parameter at the location of the test object.