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State Soil Geographic Data Base (STATSGO)Table of Contents
Background FTP STATSGO now! (2.8MB pk zip file includes DLG3 and tabular data. PC users need to do unix2dos) BackgroundThe U.S. Department of Agriculture's (USDA) Natural Resources Conservation Service is responsible for collectin, storing, maintaining, and distributing soil survey information for privately owned lands in the United States. NRCS established three soil geographic data bases representing different intensities and scales of mapping. Each data base has a common link to an data file for each map unit component. The Soil Interpretations Record (SIR) data base provides the attribute data for each geographic data base. The three soil geographic data bases are the Soil Survey Geographic (SSURGO) data base, the State Soil Geographic (STATSGO) data base, and the National Soil Geographic (NATSGO) data base. Components of map units in each data base are generally the phases of soil series that enable the most precise interpretation. Interpretations are displayed differently for each geographic data base to be consistent with differing levels of detail. The Soil Interpretation Record data base contains physical and chemical soil properties for approximately 18,000 soil series recognized in the United States. The STATSGO data base was designed primarily for regional, multi-county, river basin, State, and multi-State resource planning, management, and monitoring purposes. STATSGO data are not detailed enough to make interpretations for local areas within a county. STATSGO depicts information about soil features on or near the surface of the Earth. These data are collected as part of the National Cooperative Soil Survey (NCSS). The STATSGO soil survey product is not designed for use as a primary regulatory tool in permitting or citing decisions, but may be used as a reference source. The use of these data is not restricted and may be interpreted by organizations, agencies, units of government, or others; however, they are responsible for its appropriate application. The NRCS is not responsible for any decisions made by the regulatory bodies of other Federal, State, or local agencies based upon the interpretation of these data. The NRCS does not perform any evaluations of the soil survey maps for purposes related solely to State or local regulatory programs. Extent of CoverageThe STATSGO data base provides coverage of the conterminous U.S. at a scale of 1:250,000, except for Alaska, which will be at a scale of 1:1,000,000 when it becomes available. Processing StepsSoil map unit lines and symbols were drafted in red pencil on a mylar overlay that was punch registered to fit a mylar USGS 1:250,000-scale topographic quadrangle. A detailed edit was performed on all overlays before digitizing them. The soil delineation overlays were raster scanned at a scanning resolution of at least 0.01 inch and then converted to a vector format, or were manually digitized on a digitizing tablet with a resolution of at least 0.001 inch. Four control points corresponding to the four corners of the quadrangles were used for registration during data collection. The data sets were edge matched and merged into State coverages. A detailed edit was performed on all digital data. Map information for the STATSGO data base was captured as 1:250,000-scale USGS topographic quadrangle units in a Universal Transverse Mercator (UTM) ground-based system. The horizontal datum reference system was the North American Datum of 1927 (NAD27) which is based upon the Clarke 1866 spheroid. The topographic quadrangle units were projected into an Albers Equal Area projection and merged into statewide coverages. Projection parameters: Data ValidationFile fidelity and completeness -- STATSGO data are available for the 48 conterminous States, Hawaii, and the territory of Puerto Rico. Data are in preparation for Alaska. The STATSGO Compact Disk-Read Only Memory (CD-ROM) distributed by NRCS in October 1994 contained a corrupted data file for the COMP attribute table for New Mexico. If a user encounters the corrupted file, a corrected version can be obtained from the NRCS. Horizontal Positional Accuracy -- The horizontal position of these digital data is based upon their compilation to base maps that meet National Map Accuracy Standards. The difference in positional accuracy between the map unit boundaries in the field and their digitized map locations is unknown. The locational accuracy of soil delineations on the ground varies with the transition between map units. For example, in areas where changes in soils, climate, topography, and geology occur subtly across a portion of a State, the transition between soil map unit boundaries will be gradual. Where these features change abruptly, such as from an area of foothills to a lake plain, the transition zone will be very narrow. Soil delineation boundaries were digitized within 0.01 inch of their locations on the digitizing source. The digital map elements are edge matched between data sets. The data along each State boundary are matched against the data for the adjacent State. Edge locations generally do not deviate from centerline to centerline by more than 0.01 inch. Attribute accuracy -- Attribute data for some data elements may be incomplete or missing. Where data are unavailable, a mask should be used to exclude the area from analysis. Attribute accuracy is tested by manual comparison of the source with hard copy plots and/or symbolized display of the map data on an interactive computer graphic system. Selected attributes that cannot be visually verified on plots or on screen are interactively queried and verified on screen. In addition, the attributes are tested against a master set of valid attributes. All attribute data conform to the attribute codes in the classification and correlation document. Amendments are current as of the date of digitizing. Data CharacteristicsA map unit is a collection of areas defined in terms of similar patterns of soils and/or nonsoil areas. Each map unit differs from the other units in a survey area and is uniquely identified. One or more individual areas (delineations or polygons) can be labeled with a given map unit identification code. Each map unit consists of one to 21 components. In those few areas where detailed maps do not exist, reconnaissance soil surveys are combined with data on geology, topography, vegetation, climate, and remote sensing images to delineate map units and to estimate the percentages of components. The STATSGO map unit components are soil series phases, and their percent composition represents the estimated areal proportion of each component within the STATSGO map unit. The composition for a map unit is generalized to represent the State extent of that map unit and not the extent of any single map unit delineation. These specifications provide a nationally consistent representation of STATSGO attribute data. The actual composition and interpretive purity of the map unit delineations were based on statistical analysis of transect data. The composition was largely determined by measuring transects on detailed soil survey maps. The number of transects used was proportional to the relative size, number, and complexity of the delineations. The combined data on the length of the map units crossed by the transects were used to determine the percentages of the different soil and nonsoil areas in each map unit. Specific limits were established on the classification of soils and the design and name of map units. See the References section for publications that describe these limits. Spatial ResolutionThe minimum area of delineation is approximately 625 hectares (1,544 acres), which is represented on a 1:250,000-scale map by an area approximately 1 cm by 1 cm (0.4 inch by 0.4 inch). Linear delineations are not less than 0.5 cm (0.2 inch) in width. Typically, there are 100 to 200 delineations per 1:250,000-scale quadrangle, but these numbers may increase up to 400 delineations per quadrangle. Delineations depict the dominant soils that make up the landscape. Data OrganizationSoil scientists have developed the concept of a soil association to describe mixtures or patterns of soils that occur together on the landscape. The concept of a Map Unit is used to implement the soil association concept in the STATSGO data base. A Map Unit represents a collection of soils. The individual soils in the collection may be similar to each other, or may be very different from each other. In some cases, the collection includes non-soil areas such as bedrock, gravel pits, or water. The individual soils or non-soil areas in the collection are referred to as Components of the Map Unit. Each Map Unit is uniquely identified by a Map Unit ID. Map Unit Delineations are closed polygons on the map. More than one Map Unit Delineation can be labeled with a given Map Unit ID. The attributes of the Map Unit summarize the range of soil characteristics that occur in the Map Unit, and thus are representative of a State or multi-State region in which the Map Unit occurs, and are not specific to a single Map Unit Delineation. A set of relational tables is used to store the attributes for the Map Unit and its Components. The attribute data base gives the proportionate extent of the component soils and the properties for each soil. The data base contains data on the physical and chemical soil properties, and soil interpretations for engineering, water management, recreation, agronomic, woodland, range, and wildlife uses of the soil. The data base consists of the following relational tables:
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