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...

Answer 1

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.