1. A method for coordinate generation to be implemented using an orientation device that is provided with a sensor, said method comprising the steps of: A) providing at least four reference marks on a target; B) aiming the orientation device at a target point on the target, and operating the orientation device such that the sensor is able to capture an image of the target that contains the reference marks; C1) computing coordinates of the reference marks in the image captured in step B) to obtain vector combinations; C2) finding a pair of most parallel vectors from the vector combinations, and obtaining absolute coordinates of the reference marks projected onto the coordinate space of the target according to the pair of most parallel vectors; and D) determining relative coordinates of the target point in a coordinate space of the target with reference to the absolute coordinates of the reference marks assigned in step C2); E) aiming the orientation device at an arbitrary point on the target; and F) operating the orientation device such that the sensor is able to capture an image of the target that contains the reference marks; wherein step D) includes the sub-steps of: D1) determining coordinates of the reference marks from the image captured in step F); D2) performing projective transformation calculations upon the coordinates of the reference marks determined in sub-step Dl) to obtain a transformation matrix; D3) correcting an image-capturing axis of the sensor of the orientation device with reference to the transformation matrix obtained in sub-step D2) and an established coordinate space relation between the target and the orientation device; and D4) determining the relative coordinates of the target point with reference to the corrected image-capturing axis.