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 (STATSGO) is an older, 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 that is not yet available for the entire country, although national coverage is planned. 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 STATSGO 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 STATSGO 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. This metadata refers specifically
to the Delaware Statsgo data, but information for the other states will be similar.
Soils information FTP site - Soils data from the Statsgo system for every state except Alaska, in multiple formats. Contact information is given for Alaska data. The DOS Arc/Info data will not work with PC Arc/Info, but will work with ArcView. You need to create a unique subdirectory to put the state files in before unzipping them, as the data is not in Arc Export format, but is actually the data files (pat, log, various dbf files, etc) zipped together.
SSURGO Data Site - includes documentation, status maps, and SSURGO data (DLG format on the web site, Arc/Info export format for some areas by special request).
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. (MUIR Internet Site). 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
Statsgo: As noted above, the data is in Arc/Info format, with the files all zipped together. Although a notation at the site says the data cannot be used by PC Arc/Info due to the presence of a separate Info directory, that was not seen in the data tested. The attribute tables listed above downloaded as .dbf files, and were readily imported into a variety of programs.
Ssurgo: The data downloaded was in a large zipped file, which when decompressed had multiple quads of data in it. There was at least one DLG spatial file for each quad; some had the additional special features DLG spatial file. The DLG script file for ArcView, described earlier for the USGS DLG data, worked fine on this data as well. The soil data (generally a large file ending with .0AF) is a polygon file. The special features file, if present, will be a point or line file ending with .0SF.
The attribute tables listed above came down as ASCII fixed length text files, but with no .txt extension. They can be imported into ArcView by either adding the extension, or typing the name of the file (for example forest) in the file open box.
In addition, there is a DLG companion attribute file for each quad that assists in linking between the major/minor codes in the DLG spatial data files and the attribute files listed above. For the soils data these files end with .0AA, and for the special features data they end with .0SA. The beginning of the files will have the same letters as the related DLG files (for example s4509901). These files are ASCII text, delimited, with ID, major, minor and muid as the fields. They can be imported directly into ArcView by selecting *.txt as the file type and typing in the name, but you will not be able to edit the table at all that way. A better method is to import the file into another software program (such as Microsoft Access) and export it as a DBF file. You will need to edit the last field (MUID) either in ArcView or another program, so this is important.
The values in the MUID field need to be customized for each quad in order to be used as a lookup field. For example, in the North Dakota McIntosh County data, 510000 had to be added to each value in the MUID field for each quad so that the MUID field in the .0AA files and the muid field in the attribute files would match. This number is directly related to the Soil Survey Area ID, and will vary with the data you download. It should be consistent over any given set of data downloaded together however. The field you are editing actually contains the musym value; the muid is the numeric soil survey value plus the musym number.
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.