Abstract.Assuming that other sources of error can be neglected, the reliability of a land suitability classification depends on the homogeneity of physiographically delineated map units with regard to land qualities. The map unit homogeneity of a small area in France was estimated using 64 observation points, arranged according to a nested sampling scheme, followed by nested analysis of variance.The analysis shows that in this area map units are too heterogeneous to accept the suitability classification as being completely reliable. However, alternative procedures using methods of optimal interpolation to map gradual change within the physiographic units are too expensive at a mapping scale of 1:25000 or smaller. It is not possible to produce completely accurate suitability maps at smaller scales. However, incorporating nested sampling and analysis of variance as standard procedures in land evaluation surveys costs little effort and yields at least an estimate of map accuracy and reliability of the suitability classification.