[postgis-users] Simplifying a multipolygon layer, keeping polygon connection

[Sandro Santilli]

On Mon, Apr 09, 2012 at 07:33:41PM +0200, Nicolas Ribot wrote:
> Hmm much easier with Postgis Topology...
> 
> (thanks Brent for the Topology example)
> 
> By the way, is it the right way to use topology to simplify a layer ?

It's one way, there may be multiple ones.
Add this new way to to wiki ?

It'd be interesting to see some banchmarks of the two methods you reported.

I'd point out that the toTopoGeom function does the association with you,
directly giving you a TopoGeometry object back. Brent didn't use TopoGeometry
at all, which is legit, but that's the intended mean to associate attributes
to topologically-defined locations.

I've been actually thinking that an ST_Simplify() override taking a
TopoGeometry input might be a good idea. You would be simplifying the same
up to twice each, but the usage would be simper.

--strk;

> 
> create table new_dept as (
> 	select gid, code_dept, (st_dump(geom)).*
> 	from departement
> );
> create index new_dept_geom_gist on new_dept using gist(geom);
> 
> -- create new empty topology structure
> select CreateTopology('topo1',2154,0);
> 
> -- add all departements polygons to topology in one operation as a collection
> select ST_CreateTopoGeo('topo1',ST_Collect(geom))
> from departement;
> 
> -- create a new topo based on the simplification of existing one
> -- should not be the right way to do it, but calling ST_ChangeEdgeGeom
> failed with a simplify linestring
> select CreateTopology('topo2',2154,0);
> 
> select ST_CreateTopoGeo('topo2', geom)
> from (
> 	select ST_Collect(st_simplifyPreserveTopology(geom, 10000)) as geom
> 	from topo1.edge_data
> ) as foo;
> 
> -- associates new simplified faces with points on surface, in the new_dept table
> alter table new_dept add column simple_geom geometry(POLYGON, 2154);
> 
> -- retrieves polygons by comparing surfaces (pip is not enough for
> odd-shaped polygons)
> with simple_face as (
> 	select st_getFaceGeometry('topo2', face_id) as geom
> 	from topo2.face
> 	where face_id > 0
> ) update new_dept d set simple_geom = sf.geom
> from simple_face sf
> where st_intersects(d.geom, sf.geom)
> and st_area(st_intersection(sf.geom, d.geom))/st_area(sf.geom) > 0.5;
> 
> Nicolas
> 
> On 9 April 2012 01:02, Nicolas Ribot <nicolas.ribot at gmail.com> wrote:
> > For those interested, I've added an entry to the postgis wiki showing
> > an example of polygon layer simplification:
> >
> > http://trac.osgeo.org/postgis/wiki/UsersWikiSimplifyPreserveTopology
> >
> > Nicolas
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