Documentation and Tools for Utilizing NRCS Soils Data
The National resource Conservation Service (NRCS) soils data sets depict information about soil features on or near the surface of the Earth. The data were collected as part of the National Cooperative Soil Survey. There are actually two separate spatial data sets discussed here. The State Soil Geographic Data base (STATSGO2) is a small-scale data set (1:250,000 for most of the country, 1;1,000,000 for Alaska). The Soil Survey Geographic Data Base (SSURGO) is a newer, large scale (1:24,000) data set. SSURGO is a more detailed, updated version of STATSGO, but the underlying classification systems and databases are very similar. Differences between the two will be noted where applicable.
The STATSGO2 data set consists of a broad based inventory of soil and nonsoil areas that occur in a repeatable pattern on the landscape and that can be cartographically shown at the scale mapped. The soil maps for STATSGO2 are compiled by generalizing more detailed soil survey maps. Where more detailed soil survey maps are note available, data on geology, topography, vegetation, and climate are assembled, together with Land Remote Sensing Satellite (LANDSAT) images. Map unit composition is determined by sampling areas on the more detailed maps and expanding the data statistically to characterize the whole map unit. The map data are collected in 1- by 2- degree topographic quadrangle units, merged, and distributed as statewide coverages.
The SSURGO data set is more detailed and consists of georeferenced digital map data and computerized attribute data. The map data are in a 7.5 minute quadrangle format and include a detailed, field verified inventory of soils and nonsoil areas that normally occur in a repeatable pattern on the landscape and that can be cartographically shown at the scale mapped. Sometimes a special soil features layer (point and line features) is included. This layer displays the location of features too small to delineate at the mapping scale, but that are large enough and contrasting enough to significantly influence use and management. The soil map units are linked to attributes in the Map Unit Interpretations Record relational data base, which gives the proportionate extent of the component soils and their properties.
The Federal Geographic Data Committee (FGDC) has finalized a national soils data standard. The complete standard can be downloaded from the FGDC site. It is a large and complex document. While the underlying data definitions will be the same as the NRCS data, there will be differences in the naming of the elements and linkages between the tables. NRCS had indicated that they will bring the SSURGO data set into compliance with the FGDC standard; they are unsure if they will go back and redo STATSGO as well. The underlying data will still be the same; the major impact if both data sets are not updated would be difficulty in comparing data or linking data.
Soils Documentation and Data Sources
Metadata for Statsgo data. Metadata for the USGS generalization available on Data.gov.
STATSGO2 Data on Data.gov - Generalized soils data from the Statsgo system for every state except Alaska, in raster format, 1km grid size, with attribute tables that can be joined to the grid. (From USGS)
SSURGO Data on the USDA NRCS Data Gateway - Select data in shapefile and tabular format by county. Data is free, but requires registration. SSURGO Data on the USDA NRCS Web Soil Survey - Mapper interface that allows the user to select and download shapefiles and tabular data by defining an Area of Interest
General Data Use Information for SSURGO data, in zipped MS Word 5.1 format. This document is also available from the site above, but is combined with the appendices and is therefore much larger.
Appendices for SSURGO Use Information Appendix A includes the definition of the soil data elements, Appendix B is the Definition of the soil data codes, and Appendix C is a value table. These are also in zipped MS Word 5.1 format.
Tools for Using Soils Data
Database notes Both data sets use the Map Units Interpretations relational data base (MUIR) to describe the soil properties depicted. Generally the column in the tables labeled muid is the basic map unit (polygon) that depicts the soil classes. The biggest difference between the data sets is the scale of the maps and the related accuracy of the underlying data. There is also some additional data included in the SSURGO data set, in the form of added attribute tables. The following list describes the attribute tables found in the SSURGO and STATSGO data sets; those items with an * are found only in SSURGO. For additional documentation on these tables or the MUIR database, visit the WWW site above, and go to the schema and dictionary section.
- codes (data base codes) - stores information on all codes used in the data base
- comp (map unit component) - stores information on soil map unit components
- compyld (component crop yield) - stores crop yield information for soil map unit components
- forest (forest understory) - stores information for plant cover as forest understory for soil map unit components.
- * helclass (highly erodible lands class) - stores the highly erodible land classification for wind and water assigned to the soil map units.
- * hydcomp (hydric component information) - stores data related to the hydric classification, criteria, landform, etc.
- * inclusn (map unit inclusion) - stores the names of soils included in the soil map units
- interp (interpretation) - stores soil interpretation ratings (both limitation ratings and suitability ratings) for soil map unit components
- layer (soil layer) - stores characteristics of soil layers for soil map unit components
- mapunit (map unit) - stores information that applies to all components of a soil map unit
- * mucoacre (map unit county acres) - stores the number of acres for the map unit within a county
- *muyld (map unit yield) - stores crop yield information for the soil map unit
- plantcom (plant composition) - stores plant symbols and percent of plant composition associated with components of a soil map unit
- plantnm (plant name) - stores the common and scientific names for plants used in the data base
- *rangenm (range name) - stores the range site names.
- rsprod (range site production) - stores range site production information for soil map unit components
- *ssacoac (soil survey area county acreage) - stores the acreage for the county within the boundary of the soil survey area
- *ssarea (soil survey area) - stores information that will apply to an entire soil survey area
- taxclass (taxonomic classification) - stores the taxonomic classification for soils in the data base
- windbrk (windbreak) - stores information on recommended windbreak plants for soil map unit components
- wlhabit (wildlife habitat) - stores wildlife habitat information for soil map unit components
- woodland (woodland) - stores information on common indicator trees for soil map unit components.
- woodmgt (woodland management) - stores woodland management information for soil map unit components.
- yldunits (yield units) - stores crop names and the units used to measure yield
Data Use notes
Field Names: There are a couple of field names that are most useful in linking the attribute tables. The field muid is the basic unit for soil information, and can be found in almost all the tables. It can be used to link to the codes attribute table, to find information about the soil types. The field musym also links to that basic unit (see above). Finally, the field plantsym is used with the plantnm table, and assists in relating the names of the plants with the plant codes used in tables such as forest or windbrk.