comparison unit 104 and the generation unit 105 first the u201c relative position u201d and the u201c relative passing direction u201d before and after a change in the target section are calculated s 31 in step s 31 when the u201c relative position u201d and the u201c relative passing direction u201d of the target section before the change are stored in memory etc only the u201c relative position u201d and the u201c relative passing direction u201d after the change are calculated next with respect to the u201c relative position u201d and the u201c relative passing direction u201d of the target section the difference between a value after a change and a value before the change is calculated s 32 it is determined whether or not the difference is equal to or larger than a predetermined value s 33 if it is equal to or larger than the predetermined value control is passed to step s 37 on the other hand if it is smaller than the predetermined value control is passed to step s 34 in step s 34 the gravitational consideration flag of the target section is checked and it is determined whether or not gravity is to be considered in the determination it is determined that gravity is to be considered if the gravitational consideration flag of the target section is set in the on position and it is determined that gravity is not to be considered if the gravitational consideration flag of the target section is set in the off position if it is determined in step s 34 that.