Disclosed herein is a method for identifying a constellation of alignment marks within an arrangement of computer readable marks in an image (1412), the arrangement of computer readable marks (1412) including alignment marks (1401-1406) and data carrying marks (1407-1410). The alignment marks (1401-1406) define a reference grid within the arrangement of computer readable marks and the data carrying marks (1407-1410) are modulated with respect to the reference grid to encode data. The method selects at least two marks (1501, 1502) from the arrangement of computer readable marks and determines a rotation center with reference to the selected marks (1501, 1502). The method then determines rotated positions for the selected marks by rotating each of the selected marks by a predetermined angle about the rotation center. For each rotated position, a mark having a position that is less than a predetermined threshold from the rotated position is located from the arrangement of marks and then the selected marks and the located marks are identified as the constellation of alignment marks.