1. a Center for Spatial Information Science , University of Tokyo , 7‐3‐1 Hongo, Bunkyo‐ku, Tokyo 113‐8656, Japan
2. b PASCO Corp. , 2‐8‐10 Higashiyama, Meguro‐ku, Tokyo, Japan
3. c Department of Mathematical Informatics , University of Tokyo , 7‐3‐1 Hongo, Bunkyo‐ku, Tokyo 113‐8656, Japan