Mastering PostGIS

First, we will add new features to the topology using two different methods. One approach is to use the standard ST_ functions. We will start with two isolated nodes. This is done with the topology.ST_AddIsoNode function. It takes three arguments: the topology name, the containing face ID (in our case, we will create nodes in empty space, so it will be NULL), and the point geometry:

SELECT topology.ST_AddIsoNode('my_topology',NULL,'SRID=4326;POINT(-1 1)'); 
 st_addisonode 
--------------- 
          4568 

SELECT topology.ST_AddIsoNode('my_topology',NULL,'SRID=4326;POINT(1 1)'); 
 st_addisonode 
--------------- 
          4569

Note the returned nodes' IDs, as they will be necessary later when adding edges:

Two isolated nodes added to a topology

Next, it's time to add two edges, the first as isolated, the second creating a new face. The edge creation functions ST_AddIsoEdge and ST_AddEdgeNewFaces require four arguments: the topology name, the start node ID, the end node ID, and a LINESTRING geometry:

SELECT topology.ST_AddIsoEdge('my_topology',4568,4569,'SRID=4326;LINESTRING(-1 1, 1 1)'); 
 st_addisoedge 
--------------- 
          4766

New isolated edge

We will close the polygon shape now. As the next edge will be bound to another edge, it cannot be called isolated, and therefore another function is necessary:

          
SELECT topology.ST_AddEdgeNewFaces('my_topology',4569,4568,'SRID=4326;LINESTRING(1 1, 1 -1, -1 -1, -1 1)'); 
 st_addedgenewfaces 
-------------------- 
               4767

New closed edge

Now we can verify the newly added elements (and find out the face ID):

select edge_id, start_node, end_node, left_face, right_face 
from my_topology.edge_data where edge_id in(4766,4767); 
 edge_id | start_node | end_node | left_face | right_face 
---------+------------+----------+-----------+------------ 
    4766 |       4568 |     4569 |         0 |       4258 
    4767 |       4569 |     4568 |         0 |       4258 
(2 rows)

The following is the face geometry:

SELECT ST_AsText(ST_GetFaceGeometry('my_topology',4258)); 
              st_astext               
------------------------------------- 
 POLYGON((-1 1,1 1,1 -1,-1 -1,-1 1))

Newly created face

Another option is to use a PostGIS-specific function: TopoGeo_AddPolygon. When using it, we don't have to think about creating nodes and edges, as they will be created automatically. The function takes three arguments: the topology name, the polygon geometry (it has to be valid, and in the same SRID as topology), and a snapping tolerance when splitting existing edges:

SELECT topology.TopoGeo_AddPolygon('my_topology','SRID=4326;POLYGON((-2 1, -1.5 1, -1.5 -1, -2 -1, -2 1))',0.002); 
 topogeo_addpolygon 
-------------------- 
               4259

In this case, a new polygon was added in empty space. One closed edge has been created, as well as one node connecting its terminals. The newly created face ID was returned:

Newly created nodes and edges after the TopoGeo_AddPolygon function

Table of Contents for
Mastering PostGIS

Adding new elements

Table of Contents for Mastering PostGIS

Table of Contents for
Mastering PostGIS