[image: test_withgap.png] [image: test_withoutgap.png]
On Wed, Dec 7, 2022 at 11:13 AM Douglas Fan <douglas.mh...@gmail.com> wrote: > Hi all, > > Earlier I have asked a question about topology here > <https://gis.stackexchange.com/questions/446736/postgis-topology-result-still-has-gaps-and-overlap?noredirect=1#comment729180_446736>. > I still cannot understand the mechanism behind it so I hope that I may get > some help here. > > I am trying to build topology on PostGIS with 4000+ polygons. The input > are not clean, with overlaps and gaps. I have created a procedure to commit > each update statement for each polygon so that I can catch the exception > and continue the process. The result still contain gaps and overlaps that > are smaller than the tolerance and expected to be snapped. > > I have also tried to run toTopoGeom in the order of the number of vertices > of each polygon, with the idea that creating polygons with more vertices > may help the snapping process. It doesn't solve the problem and the number > of slivers is almost the same. However I have noticed that sequence of the > polygon to run toTopoGeom changes the result. > > Use these 3 polygons as an example. When I create 3303 first, then 2451, > then 944, the gap cannot be cleaned. Yet when I create it in the reversed > order(944->2451->3303), the gap is gone. The gap is also gone when I run > toTopoGeom with only these 3 polygons together. (not one by one) > > [image: test_withgap.png] > > [image: test_withoutgap.png] > > Does anyone have experience with this issue and may have an idea to > explain it? It seems to me an extra vertex on the horizontal edge of 3303 > must be provided so that the corner of 944 and 2451 can be snapped to it, > but I have not figured out a way to create such vertices for each T shape > or ⊥ shape scenario. > > Thank you for your time! > > Regards, > Douglas >
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