The distinction between the performance of a classifier and the accura
cy of the resulting thematic map is discussed. Sampling the map itself
is recommended as the preferred method for testing its accuracy. It i
s shown, nonetheless, that map accuracy can also be determined from th
e performance of a classifier, as assessed from testing data, provided
the prior probabilities of class membership are well estimated. Unfor
tunately, that is rarefy done in practice so that errors are often mad
e in reported map accuracy figures if they are inferred from classifie
r behavior. Examples are presented to illustrate how an apparently wel
l-behaved classifier can give poor results for certain classes, becaus
e of the proportions of those classes in the region being imaged.