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Disaggregation of land types using terrain analysis, expert knowledge and GIS methods

Soil maps’ value is increasingly recognised for enabling the optimal management of ecosystems. Digital soil mapping (DSM) can overcome the cost constraints of traditional mapping methods, but requires local area-specific research. As South Africa is blessed with the land type survey, local DSM research should start with the disaggregation of this resource. This paper shows how two land types (Ea34 and Ca11) near Newcastle in KwaZulu-Natal were disaggregated using DSM methods. A series of soil maps were created. With each map, more information was incorporated when creating the map. For Map 1 only the land type inventory and terrain analysis were used. A reconnaissance field visit with the land type surveyor was added for the second map. Field work and a simplified soil association legend improved the map accuracy for Maps 3 and 4, which were created using 30% and 60%, respectively, of the observation points as training data. The accuracy of the maps increased when more information was utilised. Map 1 reached an accuracy of 35%, whereas Map 4 achieved a commendable accuracy of 67%. Thus DSM methods can be used to disaggregate land types into accurate soil association maps. Emerging principles include that lithology rather than hard geology should be used as parent material input, field work is critical to obtain acceptable results, and simplifying the map legend into soil associations improves the accuracy of the map.