[postgis-users] Topology: cannot delete slivers (or gaps)

[Humberto Cereser Ibanez]

Hi Rémi,

On Wed, 2014-11-12 at 10:51 +0100, Rémi Cura wrote:
> Note you could have directly exported into postgis topology, which is
> slow but very convenient (stil faster than postgis topology
> conversion).
> 
> I'm not a grass user,
> 
> from what I understood you have to use the "-l" switch in the command
>  "v.out.postgis"
Thanks for your tip. It will be useful for make good use of Postgis
topology.
> 
> 
> Glad it helped =)
> 
> Cheers,
> Rémi-C

Cheers,

Humberto
> 
> 
> 2014-11-10 13:56 GMT+01:00 Humberto Cereser Ibanez
> <humberto at pastoraldacrianca.org.br>:
>         I was with the same problem, getting gaps in some adjacent
>         polygons
>         while trying to simplify polygons with Postgis.
>         Thank Guillaume / Rémi / Sandro by the contributions.
>         Illuminated by this thread, I created a script in which I
>         avoided gaps
>         from the bad polygons. Really the most appropriate tool,
>         GRASS, became
>         this work easier.
>         
>         On Fri, 2014-11-07 at 13:14 -0800, Guillaume Drolet wrote:
>         > I finally finished cleaning my topology, using a mix of SQL
>         commands,
>         > functions, and also using Sandro's QGIS topology editor
>         plugin.
>         >
>         > In parallel, I also tried going the GRASS route, importing
>         my original
>         > shapefile in GRASS and applying the cleaning function
>         (v.clean ...) as
>         > suggested by Remi. I injected the result into PostGIS and
>         created a
>         > topology. Most of the 23 slivers were there in the topology
>         so one of two
>         > things: 1) GRASS didn't fix them (maybe I didn't choose an
>         appropriate
>         > tolerance) or 2) they were created by PostGIS when building
>         the topology.
>         Simplifying polygons with QGIS and GRASS
>            Note: SRID = 4326
>         1) Open shapefile in QGIS
>         2) Create New Mapset
>             Import Extent from opened Shapefile
>         3) Check the smaller area of the polygons involved
>             select ST_Area (geo_polygon) from table order by
>         ST_Area(geo_polygon);
>             Smaller area = 0.005471
>         4) Import shapefile into GRASS
>             v.in.ogr dsn
>         = /home/humberto/shapefiles/pr/41MUE250GC_SIR.shp
>         output = prGrass snap = 0.0001 min_area = 0.004 -o
>             Settings of parameters: Snapping threshold for boundaries
>         = 0.0001
>         and Minimum size of area to be imported (squere units) = 0.004
>             Note: The 0.004 value was chosen to allow importing of the
>         smaller
>         area = 0.005 and all areas greater than it, avoiding any small
>         ring that
>         would be arising from this importing.
>         5) Simplify the Shapefile polygon imported
>             v.generalize input=prGrass at elimSlivers type=area layer=1
>         -c
>         type=boundary method=douglas threshold=0.001 look_ahead=7
>         reduction=50
>         slide=0.5 angle_thresh=3 degree_thresh=0 closeness_thresh=0
>         betweeness_thresh=0 alpha=1.0 beta=1.0 iterations=1
>         output=prGrassGeneralized
>             Settings of parameters: threshold = 0.001
>             Note: the threshold value 0.001 was chosen by trial
>         6) Export the simplified polygon to Postgis
>            v.out.ogr.pg.py input=prGrassGeneralized at elimSlivers
>         type=area
>         layer=1 olayer=prgeneralized database=simp22 host=127.0.0.1
>         port=5432
>         user=name password=nonono
>         
>         >
>         > Anyway, I got what I finally got what I need, thanks a lot
>         to all of you who
>         > helped me.
>         >
>         > I'm gonna ask you one last thing if I may: I want to replace
>         the original
>         > geom column (i.e. that used to build the topogeom) with a
>         topologically
>         > correct one, and keep all the associated ecological
>         attributes. Is this the
>         > right way to do it:
>         >
>         > UPDATE syshiera.de_20k SET geom = topogeom::geometry; ?
>         >
>         > Thanks.
>         >
>         >
>         >
>         > --
>         > View this message in context:
>         http://postgis.17.x6.nabble.com/Topology-cannot-delete-slivers-or-gaps-tp5007250p5007286.html
>         > Sent from the PostGIS - User mailing list archive at
>         Nabble.com.
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>         >
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>         
>         Thanks,
>         
>         Humberto
>         
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