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

Sandro Santilli strk at keybit.net
Wed Apr 11 01:15:03 PDT 2012

On Wed, Apr 11, 2012 at 10:01:12AM +0200, Nicolas Ribot wrote:
> On 10 April 2012 12:04, Sandro Santilli <strk at keybit.net> wrote:
> > 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 ?
> Done: http://trac.osgeo.org/postgis/wiki/UsersWikiSimplifyWithTopologyExt

Great, thanks.

> > It'd be interesting to see some banchmarks of the two methods you reported.
> Just som rough figures from my test:
> Using first method, with a plpgsql function and spatial operators:
>     • it takes ~ 7.5 second to process 96 multipolygons containing ~
> 47000 vertices.
>     • it takes ~ 9 minutes to process the 36610 communes
> (=municipalities), containing more than 512000 vertices
> With Topology:
>    • treating departements took 10.2 seconds on average.
>    • I canceled the CreateTopoGeo after about 25 minutes.

What you mean by "treating departments" ? Is that the 96 multipolygons
with ~47000 vertices ?

Can you do the comparison using the same (small) input for both ?

I'm interested in speeding up the case using topology.

Note that ST_CreateTopoGeo needs to do all in memory which would be
inappropriate for large datasets, the toTopoGeom function (and the
underlying TopoGeo_addGeometry functions) would instead allow you to
populate a topology incrementally.

For sure I can tell you that topology isn't meant to be used as a 
"temporary" storage. I can imagine, for example, that you want multiple
resolutions of your data. You could build the topology once and then
generate multiple simplified versions of it, or even simplify on demand
with an ST_Simplify(TopoGeometry) [ to be implemented ].


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