Abstract
Since different pedologists will draw different soil maps of a same area, it is important to compare the differences between mapping by specialists and mapping techniques, as for example currently intensively discussed Digital Soil Mapping. Four detailed soil maps (scale 1:10.000) of a 182-ha sugarcane farm in the county of Rafard, São Paulo State, Brazil, were compared. The area has a large variation of soil formation factors. The maps were drawn independently by four soil scientists and compared with a fifth map obtained by a digital soil mapping technique. All pedologists were given the same set of information. As many field expeditions and soil pits as required by each surveyor were provided to define the mapping units (MUs). For the Digital Soil Map (DSM), spectral data were extracted from Landsat 5 Thematic Mapper (TM) imagery as well as six terrain attributes from the topographic map of the area. These data were summarized by principal component analysis to generate the map designs of groups through Fuzzy K-means clustering. Field observations were made to identify the soils in the MUs and classify them according to the Brazilian Soil Classification System (BSCS). To compare the conventional and digital (DSM) soil maps, they were crossed pairwise to generate confusion matrices that were mapped. The categorical analysis at each classification level of the BSCS showed that the agreement between the maps decreased towards the lower levels of classification and the great influence of the surveyor on both the mapping and definition of MUs in the soil map. The average correspondence between the conventional and DSM maps was similar. Therefore, the method used to obtain the DSM yielded similar results to those obtained by the conventional technique, while providing additional information about the landscape of each soil, useful for applications in future surveys of similar areas.
Highlights
In view of the huge demand of food, fibers and biofuels, agriculture must be improved both in intensity and area to supply the needs of the growing world population
The objective of this work was: to compare detailed soil maps at the same scale of the same area obtained by different pedologists; compare these maps with another map produced by a Digital Soil Map (DSM) technique; evaluate if the discrepancies between conventional maps are similar to those between conventional and digital soil maps, and determine the viability of DSM in the context of this study
Results from A and B horizons were required and the authors used a sampling grid established previously by Nanni (2000) of 100 x 100 m, sampled from the depth ranges 0-20 and 80-100 cm, representing, respectively, the A and B horizon. This information was provided from the closest possible points, never farther away than 50 m from those required by the pedologists, and always in the same physiographic and geologic setting
Summary
In view of the huge demand of food, fibers and biofuels, agriculture must be improved both in intensity and area to supply the needs of the growing world population. In this context, Brazil is one of the few countries where the area, climate, soil and relief are appropriate for this expansion (Manzatto & Assad, 2010). Soil is known to be a non-renewable resource in the short term, which is why depletion by overuse must be avoided and measures should be taken to preserve it (Hartemink & McBratney, 2008). Only 75.6 % of the national territory of Brazil is mapped at a schematic or exploratory level (scales of 1: 2,500,000 to 1:750,000), 17.1 % mapped at reconnaissance level (scales of 1:750,000 to 1:100,000) and only 0.25 % is
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