PostGIS 1.5.8 Manual - Fedora Project Packages GIT repositories

PostGIS 1.5.8 Manual98 / 310See AlsoBox2D, Box3D7.4.7 ST_ExteriorRingST_ExteriorRing — Returns a line string representing the exterior ring of the POLYGON geometry. Return NULL if the geometryis not a polygon. Will not work with MULTIPOLYGONSynopsisgeometry ST_ExteriorRing(geometry a_polygon);DescriptionReturns a line string representing the exterior ring of the POLYGON geometry. Return NULL if the geometry is not a polygon.NoteOnly works with POLYGON geometry typesThis method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. 2.1.5.1This method implements the SQL/MM specification. SQL-MM 3: 8.2.3, 8.3.3This function supports 3d and will not drop the z-index.Examples--If you have a table of polygonsSELECT gid, ST_ExteriorRing(the_geom) AS eringFROM sometable;--If you have a table of MULTIPOLYGONs--and want to return a MULTILINESTRING composed of the exterior rings of each polygonSELECT gid, ST_Collect(ST_ExteriorRing(the_geom)) AS eringsFROM (SELECT gid, (ST_Dump(the_geom)).geom As the_geomFROM sometable) As fooGROUP BY gid;--3d ExampleSELECT ST_AsEWKT(ST_ExteriorRing(ST_GeomFromEWKT(’POLYGON((0 0 1, 1 1 1, 1 2 1, 1 1 1, 0 0 1))’)));st_asewkt---------LINESTRING(0 0 1,1 1 1,1 2 1,1 1 1,0 0 1)