In a relational database, there are three different storage schemas that are defined in the standards developed by industry, government bodies, and the ISO for managing geometric objects in a relational database. 

The different geometry storage schemas have associated metadata for discovering the existence and properties of a geometry column. In the standardized specification, there is overlap between the different metadata schema, while ArcSDE has implemented the metadata schema to allow any storage schema.

An ArcSDE geometry column is called a LAYER. Layers have a number of specialized properties that are maintained by ArcSDE. These properties include:

• Layer owner, table, and column name
• Coordinate reference
• Geometry type
• Indexing parameters
• Table creation parameters
• Layer description

This information is managed in three DBMS tables called the LAYERS, SPATIAL_REF_SYS and GEOMETRY_COLUMNS tables. Three separate tables are used so that data can be normalized (in the case of spatial references) and for ease of integration with the OpenGIS Simple Features in SQL Specification.

The LAYERS table stores a row for each spatial column in the database. Applications use the layer properties to discover available spatial data sources. The layer properties are used by ArcSDE to constrain and validate the contents of the spatial column, to index geometry values, and to properly create and manage the associated DBMS tables. The entries found in the LAYERS table vary slightly depending on the underlying DBMS.

The GEOMETRY_COLUMNS table stores a row for each column of type Geometry in the database that complies with the OpenGIS SQL specification. The ArcSDE application server treats this table as write only; the only time it is accessed by the ArcSDE server is when a layer is added or deleted that uses an OpeGIS SQL data format. This table is defined by the OpenGIS SQL specification and may be updated by other applications, with geometry columns not managed by ArcSDE. When a new Geometry column is created in an OpenGIS compliant format, the fully qualified table, column name, and spatial reference ID (SRID) is added to the GEOMETRY_COLUMNS table.

Each Geometry column is associated with a spatial reference system. ArcSDE stores information on each spatial reference system in the SPATIAL_REF_SYS table. The columns of this table are those defined by the OpenGIS SQL Specification (SRID, SRTEXT, AUTH_NAME, and AUTH_SRID) and those required by ArcSDE for internal coordinate transformation. The spatial reference system identifies the coordinate system for a geometry and gives meaning to the numeric coordinate values for the Geometry.

(click on the table to view detailed schema information)