[postgis-users] Why aren't ST_ClosestPoints to polylines ST_Contained in those polylines?

Hilton Long seldomseen at scinternet.net
Sat Mar 24 12:54:37 PDT 2012

I created a table of ST_ClosestPoint()s to polylines.  I assumed that if the
closest points were on the line the queries below would give equal results,
but that's not happening.


"InsideWorking".simplified is a line.

"gaps".closeststartpoint is a point.


SELECT "gaps".closeststartpoint FROM "gaps" 

WHERE "gaps".closeststartpoint IS NOT NULL; --Query produces 462 results.


SELECT * FROM "gaps","InsideWorking" 

WHERE ST_Covers("InsideWorking".simplified,"gaps".closeststartpoint);
--Query produces 1228 results.


SELECT * FROM "gaps","InsideWorking" 

WHERE ST_Contains("InsideWorking".simplified,"gaps".closeststartpoint);
--Query produces 548 results.


SELECT "gaps".closeststartpoint FROM "gaps","InsideWorking" 

WHERE ST_Touches("InsideWorking".simplified,"gaps".closeststartpoint);
--Query produces 680 results.




SELECT "gaps".closeststartpoint FROM "gaps","InsideWorking" 

WHERE ST_DWithin("InsideWorking".simplified,"gaps".closeststartpoint,0);
--Query produces 1258 results.


Inspection shows the ST_DWithin() query is getting better results than the
other queries.  So I think I've got something that works, but I'd appreciate
some advice on why the others don't give similar results.


Thanks in advance.

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