6.4 EPSG Model and Spatial and Graph

The Oracle Spatial and Graph coordinate system support is based on, but is not always identical to, the European Petroleum Survey Group (EPSG) data model and dataset. These are described in detail at http://www.epsg.org, and the download for the EPSG geodetic parameter dataset includes a "Readme" that contains an entity-relationship (E-R) diagram. The approach taken by Oracle Spatial and Graph provides the benefits of standardization, expanded support, and flexibility:

  • The EPSG model is a comprehensive and widely accepted standard for data representation, so users familiar with it can more easily understand Spatial and Graph storage and operations.

  • Support is provided for more coordinate systems and their associated datums, ellipsoids, and projections. For example, some of the EPSG geographic and projected coordinate systems had no counterpart among coordinate systems supported for previous Spatial and Graph releases. Their addition results in an expanded set of supported coordinate systems.

  • Data transformations are more flexible. Instead of there being only one possible Oracle-defined transformation path between a given source and target coordinate system, you can specify alternative paths to be used for a specific area of applicability (that is, use case) or as the systemwide default.

    The rest of this section describes this flexibility.

For data transformations (that is, transforming data from one coordinate system to another), you can now control which transformation rules are to be applied. In previous releases, and in the current release by default, Spatial and Graph performs transformations based only on the specified source and target coordinate systems, using predetermined intermediate transformation steps. The assumption behind that default approach is that there is a single correct or preferable transformation chain.

By default, then, Spatial and Graph applies certain transformation methods for each supported transformation between specific pairs of source and target coordinate systems. For example, there are over 500 supported transformations from specific coordinate systems to the WGS 84 (longitude/latitude) coordinate system, which has the EPSG SRID value of 4326. As one example, for a transformation from SRID 4605 to SRID 4326, Spatial and Graph can use the transformation method with the COORD_OP_ID value 1445, as indicated in the SDO_COORD_OPS table (described in SDO_COORD_OPS Table), which contains one row for each transformation operation between coordinate systems.

However, you can override the default transformation by specifying a different method (from the set of Oracle-supplied methods) for the transformation for any given source and target SRID combination. You can specify a transformation as the new systemwide default, or you can associate the transformation with a named use case that can be specified when transforming a layer of spatial geometries. (A use case is simply a name given to a usage scenario or area of applicability, such as Project XYZ or Mike's Favorite Transformations; there is no relationship between use cases and database users or schemas.)

To specify a transformation as either the systemwide default or associated with a use case, use the SDO_CS.ADD_PREFERENCE_FOR_OP procedure. To remove a previously specified preference, use the SDO_CS.REVOKE_PREFERENCE_FOR_OP procedure.

When it performs a coordinate system transformation, Spatial and Graph follows these general steps to determine the specific transformation to use:

  1. If a use case has been specified, the transformation associated with that use case is applied.

  2. If no use case has been specified and if a user-defined systemwide transformation has been created for the specified source and target coordinate system pair, that transformation is applied.

  3. If no use case has been specified and if no user-defined transformation exists for the specified source and target coordinate system pair, the behavior depends on whether or not EPSG rules have been created, such as by the SDO_CS.CREATE_OBVIOUS_EPSG_RULES procedure:

    • If the EPSG rules have been created and if an EPSG rule is defined for this transformation, the EPSG transformation is applied.

    • If the EPSG rules have not been created, or if they have been created but no EPSG rule is defined for this transformation, the Oracle Spatial and Graph default transformation is applied.

To return the version number of the EPSG dataset used by Spatial and Graph, use the SDO_CS.GET_EPSG_DATA_VERSION function.