[postgis-users] St_Simplify usage with adjacent polygons

[Dane Blakely Springmeyer]

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

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