Spatial Context is a coordinate system with an identity. Any geometries that are to be spatially related must be in a common spatial context.
Providing an identify for each coordinate system supports separate workspaces, such as schematic diagrams, which are non-georeferenced. However, there are also georeferenced cases. In general, two users may create drawings using the same default spatial parameters (for example, rectangular and 10,000x10,000) that have nothing to do with each other. If their drawings are to be put into a common database, the spatial context capability of FDO preserves the container aspect of the data along wih the spatial parameters.
The FDO Spatial Context Commands are part of the FDO API. They support control over Spatial Contexts in the following ways:
- Metadata control. Creates and deletes Spatial Contexts.
- Active Spatial Context. A session setting to specify which Spatial Context to use by default while storing/retrieving geometries and performing spatial queries.
There is a default Spatial Context for each database. Its attributes (such as coordinate system) are specified when the database is created. This Spatial Context is the active one in any FDO session until a Spatial Context Command is used to change this state. The default Spatial Context’s identifier number is 0 (zero).
Spatial contexts have two tolerance attributes: XYTolerance and ZTolerance. The tolerances are in distance units that depend on the coordinate system in use. Geodetic coordinate systems typically have “on the ground” linear distance units instead of the angular (that is, degrees, minutes or seconds) units used for positional ordinates. The meter is the most common unit. Most non-geodetic systems are rectilinear and use the same unit for positional ordinates and distances, for example, meters or feet.