PostGIS Topology

PostGIS topology support is in pre-alpha stage. It consists in a topology schema model and accessory functions. A lot of working is going on in the PostGIS 2.0 release. Please refer to ​PostGIS 2.0 Topology section of manual for details.

With topology support enabled you can store topological elements, define TopoGeometry objects as being composed by these elements, convert TopoGeometry objects to simple Geometry objects to use all functions defined on the latter.

Topological elements are Faces, Edges and Nodes.

An entity-relationship diagram of the topology concepts can be found in the SVN repository of PostGIS (trunk), under the topology/ER/ directory.

The SVN repository also contains initial code, being a set of definitions and pl/pgsql functions. A README​ file therein contains usage and test notes.

Enabling PostGIS Topology

In order to enable PostGIS support you need to be equipped with the following:

• A schema-aware PostgreSQL installation (7.3 and up)
• PostGIS 1.3.x or up
• GEOS 2.1 or up

Move under the topology/ directory and run:

$ make
$ psql -f topology.sql <your_test_database>

No C libs involved so far, so no need to make install.

To uninstall/disable just drop the 'topology' schema. It contains all the objects installed by the topology.sql script (remember to cascade).

Creating a Topology

Topology data are stored in named SCHEMAs, where the topology name is the name of the SCHEMA containing its data.

A catalogue of avalable topologies is kept under the "topology"."topology" table.

To create/destroy a topology:

SELECT topology.CreateTopology(name, [srid], [tolerance]);
SELECT topology.DropTopology(name);

Loading Topology data

To load topology data in a topology you can use INSERT statements filling up the Edge, Node and Face relations under your topology schema:

• Edge
  • edge_id integer PRIMARY KEY
  • start_node integer REFERENCES Node.node_id)
  • end_node integer REFERENCES Node.node_id)
  • next_left_edge integer REFERENCES abs(Edge.edge_id)
  • next_right_edge integer REFERENCES abs(Edge.edge_id)
  • left_face integer REFERENCES Face.face_id
  • right_face integer REFERENCES Face.face_id
  • geom geometry ( a linestring )

• Node
  • node_id integer PRIMARY KEY
  • containing_face integer REFERENCES Face.face_id
  • geom geometry ( a point )

• Face
  • face_id integer PRIMARY KEY
  • mbr box2d ( can be NULL )

Details on semantic are contained in the SQL/MM specification, which this implementation follows as for these views structure.

Validating Topology

To verify validity of a topology:

SELECT * FROM topology.ValidateTopology(name);

The return set will contain references to elements involved in the invalidity.

Copying a Topology

See TopologyCopy

Editing a Topology

See TopologyEditing

Working with TopoGeometry objects

See CreatingTopoGeometryObjects

Getting simple Geometry values from TopoGeometry objects

You currently need to explicit call the TopoGeometry⇒Geometry cast function. This will probably be made implicit when the code is more tested:

SELECT topology.Geometry(TopoGeometry);

Testing Topology implementation

Tests are included under the topology/test directory. Run make w/out args to see a list of supported targets.