Rigorous, scientific geomorphology demands quantification in order that its theories can be tested. Qualitative description of surface form must therefore frequently be replaced by morphometry: the measurement of landforms. Such measurement generates nominaland ordinal-scale information (for example, qualitative landform classifications and stream ordering methods), but interval- and ratio-scale data are commonly employed. The nominal and ordinal scales are in fact often used to define 'treatments', or strata, for classification of samples of data measured on the higher scales. Some form of classification system often precedes quantitative measurement of landform attributes. For example, different indices of form may be deemed relevant for different types of sand dune, so that initial classification of such individual features as barchan, parabolic, linguoid, or sief dune forms is necessary.