Assessing the thematic accuracy of area-class soil maps

4 1 Thematic accuracy as an aspect of adequacy 5 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 Conceptual models of soil spatial variability . . . . . . . . . . . . . . 6 1.3 Geometric accuracy, symbols, and boundaries . . . . . . . . . . . . . 7 1.4 Types of thematic accuracy . . . . . . . . . . . . . . . . . . . . . . . 8 2 What is ‘agreement’ between map and ground? 9 2.1 Simple map units . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2 Compound map units . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3 Binomial tests of accuracy 12 3.1 Comparing the relative accuracy of two maps . . . . . . . . . . . . . 13 4 Multinomial tests of accuracy 14 4.1 Naïve measures of agreement . . . . . . . . . . . . . . . . . . . . . . 16 4.1.1 The weighted confusion matrix . . . . . . . . . . . . . . . . 16 4.1.2 Assigning weights . . . . . . . . . . . . . . . . . . . . . . . 20 4.2 The kappa index of agreement . . . . . . . . . . . . . . . . . . . . . 23 4.2.1 Unweighted kappa . . . . . . . . . . . . . . . . . . . . . . . 24 4.2.2 Conditional kappa . . . . . . . . . . . . . . . . . . . . . . . 25 4.2.3 Weighted kappa . . . . . . . . . . . . . . . . . . . . . . . . 26 4.3 The tau index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5 Multinomial tests applied to compound map units 29 6 Sampling schemes for accuracy assessment 32 6.1 Sample design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 6.2 Sample distribution for multinomial tests . . . . . . . . . . . . . . . . 33 6.3 Sample size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 6.3.1 Sample size for binomial tests . . . . . . . . . . . . . . . . . 34 6.3.2 Sample size for multinomial tests . . . . . . . . . . . . . . . 35 6.4 Sampling within a reference map . . . . . . . . . . . . . . . . . . . . 36