PostGIS – How to Create Polygons from Gaps

polygonpostgis

Using PostGIS, I want to fill a gap between other polygons.

Image to explain from where to extract the polygon:

Can you help with some directions what functions to use?

Best Answer

Use ST_Union to aggregate the polygons into one polygon and then ST_InteriorRingN to get the border of the gap and ST_BuildArea to get polygon of the gap. If there is more than one such gap use generate_series and ST_NumInteriorRings. Example:

WITH polygons(geom) AS
(VALUES (ST_Buffer(ST_Point(0, 0), 1.1,3)),
        (ST_Buffer(ST_Point(0, 2), 1.1,3)),
        (ST_Buffer(ST_Point(2, 2), 1.1,3)),
        (ST_Buffer(ST_Point(2, 0), 1.1,3)),
        (ST_Buffer(ST_Point(4, 1), 1.3,3))
),
bigpoly AS
(SELECT ST_UNION(geom)geom 
 FROM polygons)
SELECT ST_BuildArea(ST_InteriorRingN(geom,i)) 
FROM bigpoly
CROSS JOIN generate_series(1,(SELECT ST_NumInteriorRings(geom) FROM bigpoly)) as i;

example Blue-start polygons Purple-final polygons

Related Solutions

[GIS] Adjusting polygons to boundary and filling holes

Use ST_DumpPoints to get a set of vertices for each polygon in 3.
From that construct Voronoi Polygons around your vertices
Dissolve the Voronois by grouping
Clip the resulting shapes with your department boundary.

PostGIS – Creating Lines to Visualize Differences Between Polygon Features

Here are a few new tricks, using:

EXCEPT to remove geometries from either table that are the same, so we can focus only on geometries that are unique to each table (A_only and B_only).
ST_Snap to get exact noding for overlay operators.
Use the ST_SymDifference overlay operator to find the symmetric difference between the two geometry sets to show the differences. Update: ST_Difference shows the same result for this example. You can try either function to see what they get.

This should get what you expect:

-- CREATE OR REPLACE VIEW polygon_SymDifference AS
SELECT row_number() OVER () rn, *
FROM (
  SELECT (ST_Dump(ST_SymDifference(ST_Snap(A, B, tol), ST_Snap(B, A, tol)))).*
  FROM (
    SELECT ST_Union(DISTINCT A_only.geom) A, ST_Union(DISTINCT B_only.geom) B, 1e-5 tol
    FROM (
      SELECT ST_Boundary(geom) geom FROM polygon_a
      EXCEPT SELECT ST_Boundary(geom) geom FROM polygon_b
    ) A_only,
    (
      SELECT ST_Boundary(geom) geom FROM polygon_b
      EXCEPT SELECT ST_Boundary(geom) geom FROM polygon_a
    ) B_only
  ) s
) s;

 rn |                                        geom
----+-------------------------------------------------------------------------------------
  1 | LINESTRING(206.234028204842 -92.0360704110685,219.846021625456 -92.5340701703592)
  2 | LINESTRING(18.556700448873 -36.4496098325257,44.44438533894 -40.5104231486146)
  3 | LINESTRING(-131.974995802602 -38.6145334122719,-114.067738329597 -39.0215165366584)
(3 rows)

To unpack this answer a bit more, the first step with ST_Boundary gets the boundary of each polygon, rather than just the exterior. For instance, if there were holes, these would be traced by the boundary.

The EXCEPT clause is used to remove geometries from A that are part of B, and rows from B that are part of A. This reduces the number of rows that are part of A only, and part of B only. E.g., to get A_only:

SELECT ST_Boundary(geom) geom FROM polygon_a
EXCEPT SELECT ST_Boundary(geom) geom FROM polygon_b

Here are the 6 rows of A_only, and 3 rows of B_only:

Next, ST_Union(DISTINCT A_only.geom) is used to combine the linework into a single geometry, typically a MultiLineString.

ST_Snap is used to snap nodes from one geometry to another. For instance ST_Snap(A, B, tol) will take the A geometry, and add more nodes from the B geometry, or move them to the B geometry, if they are within tol distance. There are probably several ways to use these functions, but the idea is to get coordinates from each geometry that are exact to each other. So the two geometries after snapping look like this:

And to show differences, you can choose to use either ST_SymDifference or ST_Difference. They both show the same result for this example.

Best Answer

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PostGIS – Creating Lines to Visualize Differences Between Polygon Features

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