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How is soils data organized?

Each polygon represents a soil map unit. The map units have up to three named soil components and each component has up to six horizons.

There are some properties of map units as whole - mostly descriptive properties. Soil components have more properties and horizons have still more. Conceptually, we can think of soils data in the following way (text colors match the map unit diagram):

Map Unit

In digital soils data, the map unit information is indexed by unique map unit IDs within the state. The component information is linked to corresponding map units using the map unit IDs and is indexed with sequence numbers (from one to three) that distinguish between the three components in each map unit. So within each uniquely identified soil mapping unit there are three components that are also uniquely identified by their sequence number. Finally, within those components are horizons that are uniquely identified by their layer number. The diagram below may help you visualize the links between the data tables that you get with digital soils data.

Data Tables

The three main tables are mapunit, comp, and layer which contain data about mapping units, soil components and soil horizons respectively. These tables are linked using the muid (map unit ID) and seqnum (component sequence number) fields. As you can see, there are four other data tables that are mostly about mapping units (inclusn, helclass, muyld, and mucoacre) and nine other data tables containing component level data (wlhabit, woodmgt, interp, compyld, rsprod, woodland, windbrk, plantcom, and forest). Additionally, there are two tables that store information about the entire soil survey area (ssacoac and ssarea) and four tables that contain specialized information called lookup tables (yldunits, rangenm, plantnm, and taxclass).