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The United States Department of Agriculture - Natural Resource Conservation Service (USDA-NRCS), through the National Cooperative Soil Survey (NCSS), is in the process of developing soil geographic databases at three scales - local, regional, and national. At the regional level, the State Soil Geographic Data Base (STATSGO) has just been developed for river basin, multi-state, state, and multi-county resource planning. This database was created by generalizing soil-survey maps, including published and unpublished detailed soil surveys, county general soil maps, state general soil maps, state major land resource area maps, and, where no soil survey information was available, Landsat imagery.

STATSGO consists of georeferenced digital map data and associated digital tables of attribute data. The compiled soil maps were created with the USGS 1-degree by 2-degree topographic quadrangles (1:250,000 scale, Albers Equal Area projection) as base maps which were then merged on a state basis. Each state, with the exception of Alaska, and the Commonwealth of Puerto Rico has been mapped. The District of Columbia is included with the data for Maryland. The full STATSGO database is available on CD-ROM from the NRCS.

Map units in STATSGO are a combination of associated phases of soil series. A soil series is the lowest level in the U.S. system of taxonomy (Soil Survey Staff, 1993) and the most homogeneous with regard to properties. A phase of a soil series is based on attributes and factors that affect soil management. The composition of the map unit delineations was derived from a statistical analysis of transects across detailed soil survey maps. Percentages of the map unit components were based on the length of the map units crossed. The total number of transects was based on the size, number, and complexity of the detailed soil map delineations. The approximate minimum map unit area delineated in the STATSGO database is 625 hectares (1,544 acres). The number of delineations on each quadrangle is typically 100 to 200, but may range up to 400 (USDA, 1991).

Each STATSGO map unit may consist of as many as 21 components (a component being a phase of a soil series) which are linked to the NCSS Soil Interpretations Record (SIR) that defines soil series phases. Each component may have as many as 6 associated layers. Attributes include characteristics like: percentage of the map-unit area, surface texture and texture modifier, slope range, and flooding category, etc. Details of the exact procedures for determining map-unit composition may be found in the STATSGO Data Users Guide. STATSGO, as delivered by the NRCS, poses substantial problems for modelers not familiar with soils data or geographic information systems. Our goal in developing the 1-Km Multi-Layer Soil Characteristics Dataset was to overcome these limitations and provide the environmental modeling community with more easily accessible soils information.

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