Patent ID: 7158914

Claim:
A method comprising: measuring a test surface of a test object with respect to a reference surface to generate a first relative surface measurement, where the test surface is in a first position relative to the reference surface; measuring the test surface with respect to the reference surface to generate a second relative surface measurement, where the test surface is in a second position relative to the reference surface different from the first position; providing an estimate of a rotationally varying part of a measurement of the test surface and an estimate of a rotationally varying part of a measurement of the reference surface; calculating an estimate of a rotationally invariant part of the measurement of the test surface at a plurality of radial values based on a combination of the first relative surface measurement, the second relative surface measurement, the estimate of the rotationally varying part of the measurement of the test surface, the estimate of the rotationally varying part of the measurement of the reference surface, and a quantity representing the change between the first and second relative positions; and outputting the estimate of the rotationally invariant part of the measurement of the test surface.