A method of measuring a shape of an optical surface (14) of a test object (12) is provided. The method comprises the steps of: providing an interferometric measuring device (16) generating a measurement wave (18), arranging the interferometric measuring device (16) and the test object (12) consecutively at different measurement positions relative to each other, such that different regions (20) of the optical surface (14) are illuminated by the measurement wave (18), measuring the positional coordinates of the measuring device (16) at the different measurement positions in relation to the test object (12), obtaining surface region measurements by interferometrically measuring the wavefront of the measurement wave (18) after interaction with the respective region (20) of the optica! surface (14) using the measuring device (16) in each of the measurement positions, and determining the actual shape of the optical surface (14) by computationally combining the sub-surface measurements based on the measured positional coordinates of the interferometric measuring device (16) at each of the measurement positions.