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
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