RGeo - fix self-intersections

Question

I have a bunch of polygons that have self-intersection which causes some errors in further postprocessing them (in particular - I can't calculate intersection area of those polygons with other polygons). Here is an example of broken polygon:

{
  "type": "MultiPolygon",
  "coordinates": [
    [
      [
        [
          6.881057785381658,
          46.82373306675715
        ],
        [
          6.857171686909481,
          46.81861230543794
        ],
        [
          6.857354659059071,
          46.81856788926046
        ],
        [
          6.856993473052509,
          46.82693029065604
        ],
        [
          6.8612894138116785,
          46.83422796373707
        ],
        [
          6.86720955648855,
          46.835636765630476
        ],
        [
          6.871281147359957,
          46.83078486366309
        ],
        [
          6.871573291317274,
          46.8306215963777
        ],
        [
          6.877608228639841,
          46.82771553607934
        ],
        [
          6.877758462659651,
          46.82772313420989
        ],
        [
          6.877852632482749,
          46.827735617670285
        ],
        [
          6.880928107931434,
          46.82630213148064
        ],
        [
          6.8810399979122305,
          46.82622029042867
        ],
        [
          6.881117606743071,
          46.826115612819855
        ],
        [
          6.881057785381658,
          46.82373306675715
        ]
      ]
    ]
  ]
}

This is what it looks like on the map - as you can see, there is intersection of two polygon edges. RGeo throws an error, pointing intersection coordinate (I guess): => "Geos::GEOSException: TopologyException: Input geom 0 is invalid: Self-intersection at or near point 6.8573510795579145 46.818650764080992 at 6.8573510795579145 46.818650764080992". So, I have it at least.

My question is: is there a way to fix that intersection automatically? I read, that a possible solution is to insert 2 similar points with coordinates of self-intersection. But the problem is - the polygon has a specific order, and I don't know WHERE to insert those points.

Also, maybe there are some existing tools helping fix that...

Maybe the two points are just out of order and need to be swapped. Where does this "distortion" come from? — Stefan, Feb 14 '18 at 11:20
@Stefan thank you for your response! Points swap is less desirable solution, as it might change polygon area drastically in other cases. This data come from production DB, it's not accurate but I would like to preserve it and just fix it to make it somehow valid. — nattfodd, Feb 14 '18 at 14:15

Cody Gustafson · Answer 1 · 2018-02-21T15:44:36.270

The solution I would use is postgis's ST_MakeValid option for postgres if that is an option for you you could do something along the lines of ST_AsText(ST_MakeValid(geom_column)) or if you would rather pass in the text here is an example using the bowtie example shown in prepair:

select ST_AsText(ST_MakeValid(ST_GeomFromText('POLYGON((0 0, 0 10, 10 0, 10 10, 0 0))')));
st_astext                         
-----------------------------------------------------------
MULTIPOLYGON(((0 0,0 10,5 5,0 0)),((5 5,10 10,10 0,5 5)))
(1 row)

If that doesn't interest you, you could export those geometries and use a tool like prepair to convert them. To sum up how this works behind the scenes, it will split these "bowties" into multiple polygons which will then be made into a multipolygon. The same type of fix will applied to multipolygons.

Thank you for the answer! `prepair` tool seems what I need. The idea of splitting bowties looks good. I can't use postgis ST_MakeValid, because the data is in MongoDB :( — nattfodd, Feb 23 '18 at 09:05

RGeo - fix self-intersections

1 Answers1