polygon topology using FOSS tools

[BrianT]

Esteemed Members of The List:

I'm in the market for a FOSS workflow that replicates the "CLEAN" and
"BUILD" functionality of old ArcInfo to resolve gaps and overlaps in a
polygon layer.

The use case is that I need to aggressively simplify a variety of
polygon datasets. While your garden variety Douglas-Peucker method
can accomplish this, the tools I've seen work on each polygon
separately with no awareness of maintaining topology with surrounding
polys.

I saw some Google results that mentioned GRASS as a possible
solution. I haven't used it, but know it's now a plugin with QGIS.

At any rate, any pointers would be appreciated.


Thanks,
Brian

[Chris Helm]

Okay - this is one of our newly discovered tools that we've been simplify a lot of data with for our KML files. Its easy to use: import your shapefile, use the slider to adjust the transparency, and export the data. Presto, a simplified shapefile. Honestly I have no idea how its handles topology...

I'm somewhat glad to realize that I've forgotten all of my college course training in Arc* (Clean & Build are as foreign as "layer packages" to me these days) - its nice to achieve your goals every once in a while :)  

http://mapshaper.com/test/demo.html

Chris

--
Christopher W. Helm

National Renewable Energy Laboratory (NREL)
1617 Cole Boulevard
Golden, Colorado 80401
ph. 303.275.4639
fx: 303.275.4675

[Dane Springmeyer]

Ya, mapshaper is pretty amazing. I think mapshaper builds its own topology serverside so that it can correctly handle shared boundaries between adjacent polygons when generalizing and uses an enhanced version of the VW (Visvalingam-Whyatt) algorithm.  Some info related to the general VW algorithm here: http://users.cs.cf.ac.uk/C.B.Jones/Zhou_JonesSDH04.pdf

My understanding is for an algorithm like this to be supported in PostGIS more work would be needed on topology:

http://trac.osgeo.org/postgis/wiki/UsersWikiPostgisTopology

Dane

[BrianT]

Dane & Chris:

I played with MapShaper for a few minutes and it seems it does
preserve topology if the input file is "clean".

Which gets me halfway home; I still need something to clean-up an
original set of polygons that may be messy.

MapShaper is definitely impressive.

BT





On Nov 4, 10:58 am, Dane Springmeyer <dane.springme...@gmail.com>
wrote: