1. A method of determining a geometric transformation of an image, the method comprising: receiving an electronic image sampled from a physical object; with one or more processors: providing a seed set of candidate geometric transform parameters, the seed set being distributed over a space of candidate geometric transform parameters; for each of the candidate geometric transform parameters, transforming coordinates of a digital watermark signal with the candidate geometric transform parameters to produce a transformed watermark signal at transformed watermark signal coordinates; determining a measure of correlation between the transformed watermark signal and the electronic image, for each of the candidate geometric transform parameters; selecting a subset of the candidate geometric transform parameters based on the measure of correlation; for a selected candidate geometric transform parameter of the subset, determining peak locations within the electronic image that are within neighborhoods around the transformed watermark signal coordinates; updating locations of the transformed watermark signal coordinates to the peak locations; determining a geometric transform that maps the digital watermark signal to the updated locations; determining correlation between the electronic image signal and the digital watermark signal, transformed by the geometric transform; and refining the geometric transform by determining peak locations within the electronic image that are within a neighborhood of corresponding transformed watermark signal coordinates, updating locations of the transformed watermark signal coordinates to the peak locations, determining a new geometric transform that maps the digital watermark signal to the updated locations, and determining whether correlation has increased between the electronic image signal and the digital watermark signal, transformed by the new geometric transform.