Multiscale evaluations of global, national and regional digital soil mapping products in France

Abstract

As digital soil mapping (DSM) applications have been developed at multiple extents over the two last decades, large areas of the world are now covered by several DSM products with similar resolution and targeted soil properties. Thus, from these products, end-users must carefully select the one that will best meet their needs. The aim of this study was to evaluate three DSM products obtained at different scales (global, national and regional) over three local territories of increasing area selected in three contrasting regions of France (Alsace, Brittany and Languedoc-Roussillon). Three topsoil (5–15 cm) properties were evaluated: clay content, pH in water and soil organic carbon content. Evaluations were done at both point and soil mapping unit supports, the latter corresponding to quantitative assessment of visual accordance between DSM products and conventional local soil maps of acknowledged quality. The ability to predict soil properties well increased from global to national to regional DSM products. However, none of the DSM products tested was able to predict satisfactorily at the most local (1:25,000) scale. Evaluations of DSM products using local soil maps were generally in accordance with those using points. Evaluations using local soil maps also provided additional information about the utility of DSM products for small areas with too few soil measurements to perform punctual evaluation and for issues concerning areal-support uses of DSM products. These results suggest that when focusing on local areas, users of DSM products should evaluate their performance and, if unsatisfactory, invest in development of local DSM.