From A Large To A Small Scale Soil Map: Top-Down Against Bottom-Up Approaches. Application To The Aisne Soil Map (France)

Abstract

This paper compares two approaches for upscaling the Aisne (a 7,536 km² French department) soil database from the initial 1:25,000 nominal scale to the 1:250,000 target scale. Soil features are represented at the nominal scale, whereas pedolandscapes, which are a combination of soil-forming factors and soil variables, are required at the target scale. Because the initial soil database does not contain soil forming factor information, data on pedogenesis have to be added to the initial database. Based on the assumption that most of lithographic layers are horizontal in the area, only landform attributes are chosen to represent the soil-forming factors. Two different approaches are used to map the final pedolandscapes. The first one, called the bottom-up approach consists of classifying the soil and the landform attributes together for defining taxonomic units, which then undergo generalisation of their contours to result in pedolandscape mapping units. The second approach, called a top-down approach, consists of classifying and then mapping the landform units in order to delineate the pedolandscapes. In this paper, we focus only on the pedolandscape delineation for the target scale. The results of the two methodologies are compared to contours manually drafted by soil surveyors. The final discussion analyses the impact of taking the very detailed soil database in the Digital Soil Mapping process into account, and to give advice for digital soil mapping with limited input data.