R_180 (A): root mean square error calculated from those predicted using 180 observations by multiple regression models, I_180 (B): root mean square error calculated from spatial interpolation (IDW: inverse distance weighted) using 180 observations, R_60 (C): root mean square error calculated from those predicted using 60 observations by multiple regression models, I_60 (D): root mean square error calculated from spatial interpolation (IDW: inverse distance weighted) using 60 observations, BA: the difference in root mean square errors between those interpolated from 180 observations and those predicted using 180 observations by multiple regression models, and DC: the difference in root mean square errors between those interpolated from 180 observations and those predicted using 180 observations by multiple regression models.
