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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt'>Best to keep conversation on the list. Added back postgis-users.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>I think what might do what you want here is combination of ST_Boundary, ST_Union, and ST_Polygonize<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><ol style='margin-top:0in' start=1 type=1><li class=MsoListParagraph style='margin-left:0in;mso-list:l0 level1 lfo1'><span style='font-size:11.0pt'>Get the linework of the polygons using ST_Boundary – <a href="https://postgis.net/docs/ST_Boundary.html">https://postgis.net/docs/ST_Boundary.html</a><o:p></o:p></span></li><li class=MsoListParagraph style='margin-left:0in;mso-list:l0 level1 lfo1'><span style='font-size:11.0pt'>Union the linework – as you noted ST_Union when fed linestrings will create a multilinestring with splits at the junctions <a href="https://postgis.net/docs/ST_Union.html">https://postgis.net/docs/ST_Union.html</a><o:p></o:p></span></li><li class=MsoListParagraph style='margin-left:0in;mso-list:l0 level1 lfo1'><span style='font-size:11.0pt'>Then polygonise the linework - <a href="https://postgis.net/docs/ST_Polygonize.html">https://postgis.net/docs/ST_Polygonize.html</a> (this is an aggregate that returns a geometry collection of all the polygons formed from the linework<o:p></o:p></span></li><li class=MsoListParagraph style='margin-left:0in;mso-list:l0 level1 lfo1'><span style='font-size:11.0pt'>Use ST_Dump to dump out the individual polygons from the polygonize operation <a href="https://postgis.net/docs/ST_Dump.html">https://postgis.net/docs/ST_Dump.html</a><o:p></o:p></span></li></ol><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>WITH a(name, geom) AS ( VALUES ( 'A', ST_GeomFromText('POLYGON((0 0, 0 4, 4 4, 4 0, 0 0))') )<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'> , ( 'B', ST_GeomFromText('POLYGON((3 1, 3 3, 6 3, 6 1, 3 1))') )<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'> , ( 'C', ST_GeomFromText('POLYGON((3 -1, 3 2, 8 2, 8 -1, 3 -1))') )<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'> )<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>, b(geom) AS (SELECT ST_Union(ST_Boundary(a.geom)) AS geom FROM a)<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>SELECT (ST_Dump(ST_Polygonize( b.geom) )).geom<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>FROM b;<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'> Antonio Valanzano <anvalanz@gmail.com> <br><b>Sent:</b> Monday, December 23, 2024 6:37 AM<br><b>To:</b> Regina Obe <lr@pcorp.us><br><b>Subject:</b> Re: ST_Union behaviour<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>Regina, thanks for the fast reply.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I am unioning 4 distinct polygons(see attached polygon_overlapping.jpeg) which present some overlapping (some areas with 2 overlapping polygons, 1 area with 3 overlapping polygons) and I would like to create a new table comprising all these polygons but splitted where they overlap. (see attached new_polygons.jpeg)<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>The four initial polygons are all valid as you can see from the results of the following query<o:p></o:p></p></div><div><p class=MsoNormal>SELECT <br> id, <br> ST_isValid(geom)<br>FROM chp02.prova_overlap;<br>-- 4 rows<br>"id" "st_isvalid"<br>1 true<br>2 true<br>3 true<br>4 true<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I have also tried, as you suggested, the ST_UnaryUnion but the result is the same as ST_Union.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Antonio<o:p></o:p></p></div><div><p class=MsoNormal> <o:p></o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>Il giorno lun 23 dic 2024 alle ore 10:46 Regina Obe <<a href="mailto:lr@pcorp.us" target="_blank">lr@pcorp.us</a>> ha scritto:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'>Are you unioning one geometry or many?</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'>The only reason I can think of why ST_Union would return unchanged overlapping polygons is if you fed it a geometry collection or invalid multipolygon with overlapping polygons.</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'>In these cases you should be using ST_UnaryUnion <a href="https://postgis.net/docs/ST_UnaryUnion.html" target="_blank">https://postgis.net/docs/ST_UnaryUnion.html</a></span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt'> </span><o:p></o:p></p><div style='border:none;border-left:solid windowtext 1.5pt;padding:0in 0in 0in 4.0pt;border-color:currentcolor currentcolor currentcolor blue'><div><div style='border:none;border-top:solid windowtext 1.0pt;padding:3.0pt 0in 0in 0in;border-color:currentcolor currentcolor'><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'> Antonio Valanzano <<a href="mailto:anvalanz@gmail.com" target="_blank">anvalanz@gmail.com</a>> <br><b>Sent:</b> Monday, December 23, 2024 3:47 AM<br><b>To:</b> <a href="mailto:postgis-users@lists.osgeo.org" target="_blank">postgis-users@lists.osgeo.org</a><br><b>Subject:</b> ST_Union behaviour</span><o:p></o:p></p></div></div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Does someone know why ST_Union behaves differently with linestrings and polygons?<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>If a table contains linestrings that overlap at some points then ST_Union creates a collection of linestrings that are splitted at intersections.<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>If a table contains polygons with overlaps then ST_Union creates a collection of polygons that are still overlapped and are not splitted (the result contains only the original polygons).<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Thanks in advance.<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Antonio<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div></div></div></div></div></div></blockquote></div></div></div></body></html>