GIS databases are used to organize and process geospatial data layers. The relational database model is the most widely used model in a GIS. The relational database is a collection of tables, known also as relations, which are connected to each other by keys. The row of the table is called a tuple (or record), and the column of the table is called an attribute (or a field) that describes the spatial entity. The attributes can also be called items or variables. The tables are related to each other by using keys. The keys represent one or more attributes whose values uniquely identify a record in a table. Data in the table can be logically joined together using a key that is the common attribute data value between two tables. An example of a relational data model is shown in Figure 10.5. The key connecting zoning and parcel is the zone code, and the key connecting parcel and owner is the PID (parcel ID number). When used together, the keys can relate zoning and owner.
The relational database—simple relational tables—is queried using Structured Query Language (SQL). This is an English-like language that uses logical commands to manipulate the data stored in the relational tables. Examples of logical commands would include restrict, project, union, intersect, difference, product, join, and divide (Bolstad, 2005).
A comprehensive database used in soil surveys is the Soil Survey Geographic (SSURGO) database. The Natural Resources Conservation Service (NRCS) collects SSURGO data from field mapping activities. The data are archived in 7.5 min quadrangles for a soil survey area. Linked to SSURGO is the Map Unit Interpretations Record (MUIR) attribute database, which contains a series of tables of around eighty-eight estimated soil physical and chemical properties, interpretations, and performance data. There are three important tables in the MUIR database: the soil survey
area table, the soil survey map unit table, and the map unit components table. Keys are available for each table that relate to the other tables in the MUIR database. Additional information on the MUIR database can be found at http://soildatamart.nrcs.usda.gov/.
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