[postgis-users] How to generalize or simplify a Polygon

[Martin Davis]

I don't have time or the setup right now to generate an example that causes this, but I have no doubt that I could produce one.  Anytime a process moves line segments (e.g. by deleting points) without a global check about whether intersections are created there is the potential to corrupt the topology.  

A polygon with a inward spike (which would not be changed by DP) very close to a slightly curved line (which *would* be eliminated by DP) would exhibit this kind of failure.

Martin Davis, Senior Technical Architect
Vivid Solutions Inc.
Suite #1A-2328 Government Street   Victoria, B.C.   V8T 5G5
Phone: (250) 385 6040    Fax: (250) 385 6046
EMail: mbdavis at vividsolutions.com  Web: www.vividsolutions.com


> -----Original Message-----
> From: Matt Lynch [mailto:matt at terraEngine.com]
> Sent: Thursday, August 28, 2003 3:13 PM
> To: 'PostGIS Users Discussion'
> Subject: RE: [postgis-users] How to generalize or simplify a Polygon
> 
> 
> Hi Christoph,
> 
> I searched the postings with D-P Algorithm and found the 
> posting on Line
> Simplification that you mentioned.  I will look into that as it would
> ideal for me to have it within PostGIS.
> 
> Does anyone have any more details on the self-intersection 
> problem that
> was mentioned earlier in the thread?  I haven't been able to come up
> with a case where I could see a self-intersection generated.
> 
> Thanks,
> 
> Matt
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
> Christoph Spoerri
> Sent: Thursday, August 28, 2003 3:06 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] How to generalize or simplify a Polygon
> 
> 
> I'm not sure if this was mentioned before, but earlier this 
> summer their
> was a 
> post on the list by somebody that implemented the D-P 
> algorithm on lines
> as 
> an extension/function to PostGIS. This may be a good starting point to
> also 
> simplify polygons (if required).
> 
> Along this line, I was also think if it would be possible/useful to
> store the 
> simplified feature within the database. This would then allow the user
> to 
> specify which 'version' of a spatial dataset should be retrieved. This
> would 
> provide a similar functionality as is already available for raster
> datasets 
> where pyramids are used to provide this functionality. What does the
> rest of 
> you think about this?
> 
> Christoph
> 
> 
> On Thursday 28 August 2003 04:29 pm, Martin Davis wrote:
> > > of self-intersection when you process a polygon. So why 
> not put it 
> > > in as an method that is only a member of LineString / 
> > > MultiLineString class ?
> >
> > If we were putting D-P into JTS, that would be one place to put it, 
> > alright.  However, as I said I'd really rather keep JTS focussed on 
> > core algorithms.
> >
> > On a philosophical note, adding algorithms as methods to Geometry 
> > objects seems to be encouraged by the OO paradigm, but it's not the 
> > most scalable or flexible way of doing things.  For 
> algorithms such as
> 
> > D-P which are pretty independent of the internal 
> representation of the
> 
> > Geometry I think it's nicer to keep them outside the 
> Geometry class in
> 
> > a separate algorithm class.
> >
> > Martin Davis, Senior Technical Architect
> > Vivid Solutions Inc.
> > Suite #1A-2328 Government Street   Victoria, B.C.   V8T 5G5
> > Phone: (250) 385 6040    Fax: (250) 385 6046
> > EMail: mbdavis at vividsolutions.com  Web: www.vividsolutions.com
> >
> 
> 
> _______________________________________________
> postgis-users mailing list postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> 
> ---
> Incoming mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.507 / Virus Database: 304 - Release Date: 8/4/2003
>  
> 
> ---
> Outgoing mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.507 / Virus Database: 304 - Release Date: 8/4/2003
>  
> 
> 
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>