ST_CoverageClean
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Name

ST_CoverageClean — Computes a clean (edge matched, non-overlapping, gap-cleared) polygonal coverage, given a non-clean input.

Synopsis

geometry ST_CoverageClean(geometry winset geom, float8 gapMaximumWidth = 0, float8 snappingDistance = -1, text overlapMergeStrategy = 'MERGE_LONGEST_BORDER');

Descrizione

A window function which adjusts the edges of a set of valid polygonal geometries to produce a clean coverage. Cleaning involves:

  • snapping vertices and edges to remove small discrepancies and ensure common edges are identically noded

  • merging overlaps into a parent polygon

  • merging narrow gaps into adjacent polygons

gapMaximumWidth controls which gaps between polygons are merged. Gaps with width <= this distance are merged into an adjacent polygon.

snappingDistance controls snapping of vertices and edges. The default (-1) automatically determines a snapping distance based on the input extent. Set to 0.0 to turn off snapping.

overlapMergeStrategy specifies how overlaps are merged into a parent polygon:

  • MERGE_LONGEST_BORDER - merges into polygon with longest common border

  • MERGE_MAX_AREA - merges into polygon with maximum area

  • MERGE_MIN_AREA - merges into polygon with minimum area

  • MERGE_MIN_INDEX - merges into polygon with smallest input index (defined by order of input polygons)

The result is a clean polygonal coverage that will pass validation by ST_CoverageInvalidEdges and can be input to coverage processing functions.

[Note]
To aid in determining a maximum gap width, gaps can be computed by cleaning with gapMaximumWidth = > 0 and using ST_CoverageUnion to union the result coverage. Holes in the union correspond to gaps in the original dataset. Gap widths can be measured by extracting the holes as polygons and running ST_MaximumInscribedCircle on them; the gap width is twice the computed radius.

Availability: 3.6.0 - requires GEOS >= 3.14.0

Esempi

-- Populate input table
CREATE TABLE example AS SELECT * FROM (VALUES
  (1, 'POLYGON ((10 190, 30 160, 27 134.5, 40 110, 122 47, 120 10, 10 10, 10 190))'::geometry),
  (2, 'POLYGON ((150 190, 10 190, 30 160, 50 140, 40 110, 50 80, 130 70, 135 111, 140 130, 140 160, 150 190))'::geometry),
  (3, 'POLYGON ((140 190, 190 190, 190 80, 140 80, 140 190))'::geometry),
  (4, 'POLYGON ((190 10, 120 10, 97 77, 160 90, 170 70, 190 80, 190 10))'::geometry)
) AS v(id, geom);

Polygons with overlaps and gaps

-- Show it is an invalid coverage
SELECT ST_AsText(ST_CoverageInvalidEdges(geom) OVER ())
  FROM example;

Invalid coverage edges

-- Clean the coverage, merging gaps with width <= 1
CREATE TABLE example_clean AS
  SELECT id, ST_CoverageClean(geom, 1) OVER () AS GEOM
  FROM example;

Clean polygonal coverage, with overlaps and narrow gaps removed