Changes between Initial Version and Version 1 of UsersWikiBuildPolygonsWithLines

Timestamp:

04/08/15 20:06:42 (10 years ago)

Author:

mwtoews

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UsersWikiBuildPolygonsWithLines

@@ +1 @@
+== Building areas from lines ==
+Built-in PostGIS functions [http://postgis.net/docs/ST_BuildArea.html ST_BuildArea] and [http://postgis.net/docs/ST_Polygonize.html ST_Polygonize] can be used to construct Polygons from linework. However the start/end points of all lines must touch each other to form the polygons. Also the linework needs to be properly noded at intersections, rather than just crossing each other.
+
+A user-submitted function takes a collection of LINESTRING or a MULTILINESTRING geometry and returns a geometry:
+{{{
+#!sql
+CREATE OR REPLACE FUNCTION ST_BuildAreaLinework(linework geometry) RETURNS geometry AS
+'WITH data AS (SELECT * FROM ST_Dump($1))
+SELECT ST_SetSRID(ST_BuildArea(ST_Collect(geom)), ST_SRID($1))
+FROM
+(
+  SELECT (ST_Dump(ST_Union(geom))).geom FROM data
+) t1,
+(
+  SELECT ST_Union(ST_Intersection(A.geom, B.geom)) AS pt
+  FROM data A, data B
+  WHERE A.path[1] < B.path[1] AND ST_Intersects(A.geom, B.geom)
+) t2
+WHERE ST_Intersects(ST_StartPoint(geom), pt) AND ST_Intersects(ST_EndPoint(geom), pt);' LANGUAGE sql IMMUTABLE;
+}}}
+
+For example:
+{{{
+#!sql
+SELECT ST_BuildAreaLinework('MULTILINESTRING((36 35,45 307),(30 290,390 280),(320 60,300 310),(20 60,320 60),(120 140,168 225),(140 220,220 150,120 170))');
+}}}
+
+== External links ==
+ * [http://2010.foss4g.org/presentations/3369.pdf PostGIS: Tips for Power Users], by Paul Ramsey
+ * [http://spatialdbadvisor.com/postgis_tips_tricks/120/building-polygons-from-overlapping-linestrings-requiring-intersection Building polygons from overlapping linestrings requiring intersection], by Simon Greener