Parameter-free delineation of slope units and terrain subdivision of Italy

Abstract

Quantitative geomorphological and environmental analysis requires the adoption of well–defined spatial domains as basic mapping units. They provide local boundaries to aggregate environmental and morphometric variables and to perform calculations, thus they identify the spatial scale of the analysis. Grid cells, typically aligned with a digital elevation model, are the standard mapping unit choice. A wiser choice is represented by slope units, irregular terrain partitions delimited by drainage and divide lines that maximise geomorphological homogeneity within each unit and geomorphological heterogeneity between neighbouring units. Adoption of slope units has the advantage of enforcing a strong relation with the underlying topography, absent in grid cell–based analyses, but their objective delineation is still a challenge. A given study area admits different slope unit maps differing in number and size of units. Here, we devise an objective optimisation procedure for slope units, suitable for study areas of arbitrarily large size and with varying terrain heterogeneity. We applied the new approach to the whole of Italy, resulting in a map containing about 330,000 slope unit polygons of different sizes and shapes. The method is parameter–free due to objective optimisation using a morphometric segmentation function, and the map is readily available for general–purpose studies. A cluster analysis of slope units properties, compared with terrain elevation, slope, drainage density and lithology, confirmed that the terrain partition is geomorphologically sound. We suggest the use of the slope unit map for different terrain zonations, including landslide susceptibility modelling, hydrological and erosion modelling, geo–environmental, ecological, forestry, agriculture and land use/land cover studies requiring the identification of homogeneous terrain domains facing distinct directions.