[postgis-users] Question on topology

[Sandro Santilli]

On Thu, Jun 30, 2016 at 11:25:22AM +0200, Neumann, Andreas wrote:

> Is there a method in Postgis to create a Topology, eliminating double
> borders from the original CurvePolygons and attaching left and right
> attributes from the original polygons? I came across the Postgis
> topology module. Is this the recommended way of achieving my above goal?

I'd say YES  -- the standard ISO Topology Model involves having
"left face" and "right face" information for each edge.

> Unfortunately, there are only few ressources or examples around Postgis
> topology and from the first glimpse it looks a bit complicated to use.

The simplest path to try it would something like this:

 -- Create the topology
 SELECT CreateTopology('topo', find_srid('public','mytable','geom'));

 -- Populate the topology layer
 SELECT ST_CreateTopoGeo('topo', (
    SELECT ST_Collect(geom) FROM mytable
 ) );

Then fire up qgis, select the 'topo' schema with the DB Manager
and from the menu select Schema->TopologyViewer.

> It also looks like it can't handle CurvePolygons as input - right? I
> would have to coerce first to POLYGON geometries - right? It is ok for
> me if the circular arcs are segmentized, as this data is not for
> editing/analysis, but only for symbology purposes. 

Confirmed, yo'll have to convert to POLYGON first.

> If you have any hints or examples, please let me know! 

I guess you did read these already ?
https://strk.kbt.io/blog/tag/topology/

--strk;