1. A method for decoding an encoded point matrix image including a first set of real grid points aligned along a first coordinate axis, a second set of real grid points aligned along a second coordinate axis that intersects the first coordinate axis, and a plurality of encoded data points distributed throughout the encoded point matrix image, the method comprising the steps of: (A) locating the first and second coordinate axes according to a first predetermined principle; (B) finding a set of virtual grid points according to a second predetermined principle, each of the virtual grid points being located at an intersection of a corresponding first virtual line parallel to the first coordinate axis and a corresponding second virtual line parallel to the second coordinate axis; and (C) decoding each of the encoded data points based on positional relation thereof with a closest one of the virtual grid points found in step (B) so as to obtain information encoded in the encoded point matrix image; wherein the first predetermined principle in step (A) of the method includes: (A-1) locating the base point; (A-2) locating one of the first and second coordinate axes by finding one of the first and second sets of real grid points that are aligned with each other and with the base point; and (A-3) locating the other one of the first and second coordinate axes by finding the other one of the first and second sets of real grid points that are aligned with each other and with the base point in a direction forming an angle with said one of the first and second coordinate axes found in sub-step (A-2), the angle falling within a predetermined range.