Fig. 3. The reference soil map of seven soil classes and randomly distributed sample data sets (dense: 646 points, 2.9% of total pixels; medium: 179 points, 0.8% of total pixels; sparse: 50 points, 0.2% of total pixels). The seven soil classes (soil series) include: 1, loamy alluvium; 2, Sparta; 3, Plainfield; 4, Dakota; 5, Richwood; 6, others (e.g., water); and 7, peat and muck.