1. A model-based measurement method, comprising: utilizing an imaging device to obtain at least one optical image of a measurement site specified for a photomask; retrieving a design of photomask from a mask design database; selecting a portion of the design corresponding to the specified measurement site; utilizing a computing device implemented model of the imaging device to generate at least one simulated image of the selected portion of the design; adjusting at least one parameter of the computing device implemented model to minimize dissimilarity between said at least one simulated image and said at least one optical image, wherein said at least one parameter includes at least one of: a pattern registration parameter or a critical dimension parameter; and reporting said at least one of the pattern registration parameter or the critical dimension parameter of the computing device implemented model when the dissimilarity between said at least one simulated image and said at least one optical image is minimized.