[postgis-users] PostGIS topology behaviour with polygons

[Douglas Fan]

[image: test_withgap.png]

[image: test_withoutgap.png]

On Wed, Dec 7, 2022 at 11:13 AM Douglas Fan <douglas.mhfan at 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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