Even when areas delimited by delimiting points are not appropriate and area division has failed, success or failure of area division in such areas can easily be determined by applying a map determination condition in the map determining step (step S50). And since the areas delimited by the delimiting points are changed and the map determining step (step S50), including also steps S22 and S23 and steps S30 and S40, is repeated until the map determination condition is satisfied, the areas satisfying the map determination condition are determined appropriate and the area division in such areas can be carried out accurately. Even when there is distortion, a two-dimensional position map can be corrected accurately.