In a data region of each microdot block on a microdot matrix, a coordinate of each the microdot block on the microdot matrix is encoded into a plurality of microdots included by the data region according to an encoding method based on Reflected Gray codes. With the aid of a unique changed bit between any two consecutive Reflected Gray codes, an estimated coordinate of a frame, which is fetched by scanning the microdot matrix, on the microdot matrix, may still be decoded even if no complete microdot block is fetched within the frame.