Acomp ilation of results suggests a predictive character to the relationship overall accuracy (Y ) vs. number of classes (X) with Y ¼ 3:90X þ 86:38(r2 ¼ 0:63) for Landsat ETM+, and Y ¼ 2:78Xþ 91:69 for Ikonos (r2 ¼ 0:82) (Andrefou€et et al., in press).