Sorry for cross-posting, but I didn't get any conclusive responses on the -users mailing list so this may be more a question for the devs.<br><br>Union these 16 
linestrings, you get a topology exception.  Remove any one of them, you 
don't.  Simplifying and snapping to 
grid, at any reasonable precision, did not help my larger real data (though I 
didn't try with this little test case).<br>
<br>Is this a bug in ST_Union (or more likely, in Geos' Union)?  Isn't it supposed to be adding the node it complains is missing?<br><br>I was attempting to polygonize the linework, and in PostGIS my understanding is that the way to node the linestrings is to call ST_Union.  I eventually worked around this by importing the data into OpenJump and polygonizing there (OpenJump is built on JTS, though I don't know if it uses Geometry.Union or one of the Noder classes to do the linestring noding).<br>
<br>SELECT ST_Union(ARRAY[<br>ST_GeomFromEWKT('SRID=4326;<div id=":16s">LINESTRING(-118.30613361296
 33.8122380826708,-118.304098 33.810251,-118.304236 
33.810103,-118.304709 33.810071,-118.305765 33.810748,-118.305909 
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33.807514,-118.304845 33.807291,-118.305287 33.806817,-118.306369 
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33.803689,-118.306994 33.803306,-118.307001 33.802282,-118.306366 
33.801427,-118.304812 33.801424,-118.304852 33.800714,-118.304474 
33.799426,-118.304048 33.799149,-118.303735 33.798054,-118.302859 
33.798053,-118.302692 33.797818,-118.3043 33.796488,-118.304414 
33.796697,-118.305316 33.796699,-118.305309 33.795842,-118.305185 
33.795766,-118.307667 33.793712,-118.307871 33.79377,-118.307762 
33.794063,-118.308329 33.794165,-118.309331 33.794785,-118.309496 
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33.789477,-118.314394 33.78809,-118.314441 33.787542,-118.314939 
33.787103,-118.314933 33.787655,-118.316174 33.788043,-118.317662 
33.786051,-118.317822 33.785331,-118.31926 33.783198,-118.319515 
33.783401,-118.31952 33.784353,-118.320133 33.784875,-118.320168 
33.785781,-118.320644 33.786331,-118.321187 33.786329,-118.321664 
33.785748,-118.322465 33.786108,-118.322465 33.785588,-118.322725 
33.785508,-118.322861 33.785552,-118.322863 33.787017,-118.324012 
33.786812,-118.324484 33.78618,-118.324536 33.785867,-118.325534 
33.786479,-118.327373 33.786445,-118.32739 33.78696,-118.328084 
33.7867,-118.328192 33.786312,-118.329673 33.786315,-118.330558 
33.786692,-118.331564 33.78613,-118.332317 33.786324,-118.333303 
33.785763,-118.333296 33.7829,-118.334243 33.781681,-118.334739 
33.781502,-118.336465 33.782973,-118.337471 33.78022,-118.338271 
33.779854,-118.33873 33.779245,-118.339713 33.7811,-118.340119 
33.779571,-118.340489 33.779635,-118.340011 33.778281,-118.34055 
33.777196,-118.34009 33.777247,-118.339995 33.77709,-118.341573 
33.776763,-118.342003 33.776325,-118.342988 33.777413,-118.343422 
33.776597,-118.342638 33.77558,-118.343555 33.776151,-118.343818 
33.776151,-118.344472 33.775277,-118.344577 33.775582,-118.343992 
33.776306,-118.344084 33.77655,-118.344886 33.77729,-118.348172 
33.779425,-118.349097388008 33.7794492053201)'),<br>
ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.307700993013 
33.8134536081132,-118.307366 33.813558,-118.306908 33.812743,-118.304434
 33.810553,-118.304622 33.810418,-118.30525 33.810988,-118.305923 
33.810719,-118.306 33.810101,-118.306704 33.809973,-118.306688 
33.809247,-118.307466 33.808468,-118.307319 33.808037,-118.306547 
33.807529,-118.30539 33.807538,-118.305338 33.807293,-118.305635 
33.806943,-118.30707 33.806906,-118.307423 33.806221,-118.307375 
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33.803281,-118.307227 33.803082,-118.30738 33.802612,-118.307223 
33.802298,-118.307267 33.801238,-118.306917 33.801077,-118.305484 
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33.787281,-118.320228 33.787282,-118.320228 33.788607,-118.321767 
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33.785206,-118.343369 33.785377,-118.343965 33.78596,-118.344404 
33.785906,-118.345565 33.785188,-118.346632 33.784948,-118.346263 
33.785593,-118.345294 33.786038,-118.343339 33.787391,-118.342914 
33.788404,-118.344329 33.789551,-118.344834 33.789647,-118.345547 
33.790826,-118.346802 33.791674,-118.346973 33.792355,-118.347339 
33.79248,-118.348118 33.793207,-118.348143 33.793939,-118.348962 
33.793846,-118.349103 33.793586,-118.349468 33.793472,-118.350038 
33.792956,-118.350515 33.792864,-118.351036 33.792069,-118.353376 
33.790784,-118.355734 33.789024,-118.355245 33.788011,-118.356367 
33.786715,-118.354972 33.78611,-118.354149 33.786328,-118.354231 
33.784792,-118.354544 33.784053,-118.355962 33.784602,-118.357489 
33.785796,-118.358813435042 33.7855261326062)'),<br>
ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.304848630339 
33.8111422006911,-118.304605 33.810709,-118.303261 33.809465,-118.303066
 33.808994,-118.303047 33.808764,-118.304678 33.808727,-118.304204 
33.807417,-118.303217 33.807414,-118.303039 33.807564,-118.30284 
33.807286,-118.303155 33.80711,-118.305469 33.807118,-118.305132 
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33.785203,-118.320221 33.78533,-118.320225 33.786017,-118.320425 
33.786482,-118.321244 33.78648,-118.321721 33.785898,-118.322521 
33.78602,-118.322522 33.785558,-118.322861 33.785552,-118.322863 
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33.786393,-118.32913 33.786126,-118.330072 33.786179,-118.330661 
33.78648,-118.331149 33.786488,-118.33146 33.786208,-118.33199 
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33.779912,-118.340702 33.779767,-118.342003 33.779991,-118.342707 
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33.784214,-118.347888 33.784652,-118.348115 33.7844,-118.348775 
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33.780824,-118.352971 33.780656,-118.355227 33.782384,-118.355679 
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33.7819097651871)'),<br>
ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.357169759789 33.7834490284716,-118.357033 33.783851,-118.357898 33.784215,-118.359328629247 33.784207709798)'),<br>ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.303089803431 33.8096422063087,-118.30301 33.808914,-118.302281 33.808821,-118.302230554228 33.8087352371279)'),<br>

ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.301521804907 
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ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.357401619265 33.7838689054455,-118.357879 33.784428,-118.357962 33.785686,-118.358889500305 33.7854213315776)'),<br>ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.308746858587 33.8175427468454,-118.308708 33.817061,-118.30847770782 33.8169411415257)'),<br>

ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.307561901608 33.8160004041704,-118.307515 33.814991,-118.307658 33.814793,-118.307628474836 33.8146729847436)'),<br>ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.307382137965 33.8136716631955,-118.307365 33.813602,-118.307043979511 33.813016336587)'),<br>

ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.307042066317 
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33.784819,-118.344915 33.784888,-118.34587 33.784421,-118.347695 
33.784125,-118.348113 33.78462,-118.350786 33.78368,-118.351221 
33.783293,-118.349414 33.781941,-118.349272 33.780323,-118.351328 
33.780357,-118.352627 33.78109,-118.353084 33.77979,-118.353488 
33.779397,-118.353845 33.77931,-118.354605 33.779992,-118.354302 
33.780391,-118.354036 33.781226,-118.355018 33.780952,-118.355374 
33.781143,-118.3558 33.780502,-118.356525457447 33.7806494603301)'),<br>
ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.357238541314 33.7806010373294,-118.357325612665 33.7805398901349)'),<br>ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.357342573846 33.7805383054012,-118.357615033695 33.7806751225237)'),<br>

ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.357693277501 33.7809493965079,-118.357455525965 33.7813634913787)'),<br>ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.357444440338 33.7838848876304,-118.357453 33.783942,-118.357982 33.784141,-118.35917094071 33.7841021918463)'),<br>

ST_GeomFromEWKT('SRID=4326;LINESTRING(-118.29510929418 
33.7752042976004,-118.295895 33.775667,-118.295975 33.775514,-118.296254
 33.775883,-118.296184 33.778951,-118.296341 33.77951,-118.297119 
33.780547,-118.298343 33.780232,-118.299024 33.779858,-118.299207 
33.778491,-118.299472 33.778718,-118.301295 33.779122,-118.301733 
33.778885,-118.30297 33.778888,-118.303043 33.779769,-118.303358 
33.779732,-118.303983 33.778992,-118.303682 33.77972,-118.304919 
33.779888,-118.304667 33.780495,-118.303892 33.780511,-118.30404 
33.781208,-118.303359 33.781416,-118.304219 33.782044,-118.304214 
33.782771,-118.305056 33.782756,-118.3054 33.783381,-118.305683 
33.783381,-118.30588 33.783663,-118.305981 33.7843,-118.307116 
33.785157,-118.307545 33.785104,-118.309046 33.78565,-118.309874 
33.786831,-118.3097 33.787235,-118.309963 33.787412,-118.310398 
33.787411,-118.311112 33.78786,-118.311488 33.787548,-118.31177 
33.787632,-118.311792 33.787809,-118.311122 33.788364,-118.312091 
33.788904,-118.312363 33.789343,-118.31268 33.789485,-118.310864 
33.791643,-118.309697 33.791608,-118.309507 33.792027,-118.309862 
33.792454,-118.308861 33.79445,-118.3088 33.794935,-118.30625 
33.796639,-118.30641 33.796778,-118.305036 33.796853,-118.304479 
33.7973,-118.305042 33.797743,-118.305281 33.797743,-118.304355 
33.798502,-118.303949 33.79877,-118.303609 33.798097,-118.300308 
33.798092,-118.301358 33.800068,-118.301353 33.800585,-118.301167 
33.801237,-118.30032 33.802052,-118.29926 33.80198,-118.299237 
33.802909,-118.298648 33.804168,-118.297427 33.804616,-118.29581032417 
33.804010103814)')<br>
]) as broken;<br><br>NOTICE:  TopologyException: found non-noded 
intersection between LINESTRING (-118.323 33.7869, -118.323 33.7869) and
 LINESTRING (-118.323 33.7869, -118.323 33.7869) at -118.323 33.7869<div class="im"><br><br><br>ERROR:  GEOS union() threw an error!<br>
<br></div>********** Error **********<div class="im"><br><br>ERROR: GEOS union() threw an error!<br></div>SQL state: XX000</div><br>