1. A method for determining a displacement of a substantially rigid item relative to a frame of reference between a first time and a second time, comprising: at the first time, acquiring a first set of pointwise measurements of a physical property of the item taken at a plurality of locations fixed relative to the frame of reference; at the second time, acquiring a second set of pointwise measurements of said physical property of the item taken at said plurality of locations; and comparing a first matrix derived from said first set of pointwise measurements to a second matrix derived from said second set of pointwise measurements to determine said displacement.