[postgis-users] St_Simplify usage with adjacent polygons

[Andreas Neumann]

Hi,

yes, the Simplify() command is an implementation of the Douglas-Poiker 
algorithm. This algorithm is direction dependent, which is a problem in 
Postgis, which stores edges in polygon mosaics twice.

I've just recently discussed this issue with a colleague, who is 
currently looking into implementing an alternative simplify algorithm 
which delivers the same results regardless of directionality. The 
problem is that in a polygon mosaic one has to be sure not to remove 
nodes where 2 or more polygons join/depart. This relates to a previous 
post I asked, how to detect nodes in a polygon mosaic. The algorithm 
would have to read in a list of nodes. These nodes aren't allowed to be 
removed by the algorithm.

Another option would be to generate a topology and store the edges only 
once. There is some early work on topology in Postgis (dated from 2005), 
but I don't know if anyone is actively working on this?

Anyway, my colleague is looking into this issue. He is starting to work 
on this in a couple days. I will send a note to this list, if something 
useful is available.

If someone has other ideas how to solve the problem, I'd be happy to hear.

Andreas



Dane Blakely Springmeyer wrote:
> A question regarding the function ST_Simplify:
> 
> I have a postgis layer of watershed polygons that I would like to 
> simplify, yet while keeping all shared edges. In other words, I'd like 
> to remove vertices along shared boundaries without creating gaps between 
> adjacent polygons. I'm no expert in how the Douglas-Peuker algorithm 
> works, but I found this past postgis-users thread:
> 
> [postgis-users] Simplify() and direction dependency
> http://postgis.refractions.net/pipermail/postgis-users/2005-January/006590.html 
> 
> 
> It seems that directionality of the lines making up the multigeometry 
> may cause gaps or slivers between polygons in cases. Whether or not this 
> is the correct explanation, it IS what am experiencing. Perhaps this 
> could be avoided by fixing the input data, but short of modifying the 
> directionality of my data, are there any other functions or ideas 
> regarding the ST_simplify command that might be helpful here?
> 
> Details: Postgis 1.3.1 build on Mac Os 10.4 with postgresql 8.2.5 using 
> multipolygon data imported from shapefile.
> 
> A graphic to illustrate the slivers is attached: dark black line is raw 
> dataset, blue polygons on top are simplified using 5000 as tolerance.
> 
> Thanks,
> 
> Dane
> 
> 
> ------------------------------------------------------------------------
> 
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users

-- 

--
Andreas Neumann
Böschacherstrasse 6
CH-8624 Grüt (Gossau ZH)
Switzerland
Phone: ++41-44-2736668
Email: a.neumann at carto.net

Web: http://www.carto.net/neumann/
SVG Examples: http://www.carto.net/papers/svg/samples/
SVG.Open: http://www.svgopen.org/