Comparison between soil maps obtained by conventional and digital methods in complex area

Mapas de solos são essenciais para o desenvolvimento de áreas agrícolas de maneira economicamente viável e ambientalmente correta. Até o momento, o Brasil conta apenas com mapas de solos de escala pequena (1:250.000 a 1:1.000.000), cuja utilização é limitada para planejamento agrícola e ambiental a nível de propriedade rural. Apesar de sua importância, no Brasil poucos mapeamentos têm sido realizados pelos órgãos públicos competentes, principalmente por restrições orçamentárias e falta de pedólogos. Além disso, o 5º e 6º níveis categóricos do Sistema Brasileiro de Classificação de Solos (SiBCS), que devem ser utilizados em levantamentos detalhados, não estão definidos. Dessa forma, pouco se sabe a respeito da qualidade/similaridade dos mapas desse nível que vem sendo executados no país. Os principais motivos para essa situação são: elevado custo, mão-de-obra e tempo requeridos nos trabalhos de mapeamento. Uma alternativa para redução desses entraves é o mapeamento digital de solos (MDS). Sendo assim, objetivou-se: (i) comparar quatro mapas de solos convencionais de uma mesma área complexa, de mesma escala (1:10.000), elaborados por diferentes pedólogos de maneira independente, e (ii) comparar estes com um mapa de solos obtido por técnica de MDS. Para tanto, utilizou-se uma área de estudo de 182 ha, cultivada com cana-de-açúcar, localizada no município de Rafard (SP). Foram fornecidas aos pedólogos as mesmas informações: mapas geológico e pedológico semidetalhado (ambos escala 1:100.000), mapa planialtimétrico e fotografias aéreas (ambos escala 1:10.000). Viagens a campo, resultados de análise e exame de perfis foram realizados conforme estes julgaram necessário. Para o mapa digital de solos foram utilizados atributos de terreno (elevação, declividade, poder de fluxo dágua, fator LS, plano e perfil de curvatura), calculados a partir do modelo digital de elevação gerado com base no mapa planialtimétrico, e informação espectral superficial do solo de uma imagem de satélite Landsat 5 (sensor TM) obtida em período que a superfície do solo estava desnuda. Tais informações foram sintetizadas através da análise por componentes principais, sendo posteriormente feita uma compartimentação da área com auxílio da análise Fuzzy K-médias. Nos compartimentos foram realizadas observações para classificá-los como unidades de mapeamento. Os mapas foram então comparados dois a dois, sendo medida a correspondência espacial para cada um dos quatro níveis categóricos estabelecidos no SiBCS, e além destes acrescida a informação textural. Tanto para as comparações entre mapas convencionais, quanto para comparações com o mapa digital, houve redução da correspondência espacial para os níveis categóricos mais detalhados. A maior correspondência final (4º nível + textura) entre mapas convencionais se deu entre os mapas A e B (24,3 %). As maiores correspondências espaciais em relação ao mapa digital se deram entre este e o mapa A (35,2 %) e o mapa B (28,7 %). Portanto confirma-se que os mapas de solos convencionais variam conforme critérios subjetivos do executor, cuja mensuração é impraticável. O mapa de solos digital gerado pela metodologia proposta obteve índices de correspondência aos mapas convencionais semelhantes aos obtidos entre estes, sendo uma boa opção no processo de otimização dos trabalhos de mapeamento.Soil maps are essential for the development of agricultural areas in an economically viable and environmentally friendly manner. So far, Brazil only has soil maps of small scale (1:250,000 to 1:1,000,000), whose use is limited to agricultural and environmental planning at the farm. Despite its importance, in Brazil a few mappings have been carried out by public authorities, primarily because budget constraints and lack of soil scientists. In addition, 5 and 6 categorical levels of the Brazilian System of Soil Classification (SiBCS), which should be used for detailed surveys, are not defined. Thus, little is known about the quality and level of similarity of the maps that have been undertaken in the country. The main reasons for this situation are: high cost, manpower and time required of mapping. An alternative to reducing these barriers is the digital soil mapping (DSM). Therefore, the objectives were to: (i) compare four conventional soil maps of the same complex area of the same scale (1:10,000), prepared by different independent soil scientists independently, and (ii) compare these with a soil map obtained by the DSM technique. For this purpose, we used a study area of 182 ha cultivated with sugar cane in the municipality of Rafard (SP). The same information was provided to the soil scientists: a geological map anda a semi-detailed pedological map (both scale 1:100,000), planialtimetric map and aerial photographs (both scale 1:10,000). Field trips, results of analysis and examination soil profiles were performed as deemed necessary. The information for the digital soil map included terrain attributes (elevation, slope, stream power, LS factor, plan and profile curvature), calculated from the digital elevation model generated based on the topographic map, and data reflectance of the surface soil of a satellite image Landsat 5 (TM sensor) obtained in the period that ground was bare. Such information has been summarized by principal component analysis, and subsequently made a subdivision of the area using Fuzzy K-means analysis. Observations were made in each compartment to classify them as a mapping unit. The maps were then compared two by two, the spatial correspondence being measured for each of the four levels established in the Brazilian System of Soil Classification, and beyond the increased textural information. Both for comparisons between conventional maps, and for comparisons to the digital map, there was a reduction of spatial correspondence to the more detailed categoric levels. The longest match final (4th level + texture) occurred between conventional maps A and B (24.3%). The highest spatial correlation in relation to the digital map that occurred was between it and the map A (35.2%) and the map B (28.7%). Therefore it is confirmed that the conventional soil maps vary according to subjective criteria of the soil scientist, whose measurement is impractical. The digital soil map generated by the proposed method achieved similar rates of correspondence with the conventional maps as those obtained among these, being a good choice in the optimization process of mapping work

Comparison between detailed digital and conventional soil maps of an area with complex geology

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